As requested, I'll go through each of the twenty-two cards and explain whether and how each of them could go horribly wrong.
Since you asked for each card to be rated on a four-point scale from benign to devastating, I'll use the categories benign, unpleasant, awful, and devastating - and I'll assume that while the death or forced retirement of a single PC is devastating for them, it's ultimately benign for the campaign as a whole.
The Balance
I've included forced alignment changes in my own campaigns, and I can say that whether a forced alignment change is disruptive or not depends entirely on your group and how they handle alignment. That's a big, complicated topic, but here's the short version:
If the player who draws it is the type to feel that any forced change to their character is a terribly unfair imposition, they'll probably hate having their alignment changed from their original vision for the character; if they're the type to embrace an alignment change as a roleplaying opportunity, they'll think it's cool and have fun with it; and if they're the type to ignore alignment as much as they can get away with, they'll barely notice.
Whether one PC having a different alignment causes problems for the campaign depends on the group. It is entirely possible for PCs of opposing alignments to get along and work for a common cause - but it's also entirely possible for characters of opposing alignments to murder each other in classic paladin versus goblin fashion. Personalities are complicated, and interpersonal dynamics moreso. In the worst-case scenario, this could lead to multiple PCs dying or retiring and/or the party breaking up.
For the PCs: Benign at best, devastating at worst.
For the campaign: Benign at best, devastating at worst.
The Comet
This isn't particularly dangerous on its own, since players can simply choose to ignore it. However, it can potentially seduce players into making embarrassingly poor decisions, like saying "stand back guys, I can totally defeat this flock of manticores single-handed" or "by stepping in to save my life when I was about to die of manticores, Sweeny cheated me out of a bunch of XP! I demand he face me in a duel after he drinks this cup of totally-not-poisoned wine I thoughtfully prepared!"
Hopefully your players are probably mature and sensible enough not to make these mistakes - but in the worst-case scenario, such mistakes could easily lead to one or more PC deaths. I don't know your players, so I don't know if this would be an issue for you - but it wouldn't be one with mine, since they're mostly risk-averse and don't mind admitting it when they need help.
For the PCs: Benign at best, devastating at worst.
For the campaign: Benign.
Donjon
The PC who draws the card is stripped naked and trapped in a pokeball. This is embarrassing and inconvenient.
If the party already has access to a wish spell, or you're willing to fiat them some means of finding their imprisoned comrade, this is just a spin on the classic "go on a quest to resurrect/rescue the dead/captured party member" adventure, and the standard methods of handling that scenario will work; otherwise, the lost PC has essentially been forcibly retired.
For the PCs: Awful or devastating.
For the campaign: Benign.
Euryale
Having been hit by similar curses as a player, I can confidently say that slightly worsened saving throws is a minor inconvenience at worst. Yes, they make a character worse at surviving, but in practice they're no more bad for the PC's survival than a run of bad rolls, which could have happened anyway.
For the PCs: Unpleasant.
For the campaign: Benign.
The Fates
I must admit, I've never actually had this particular effect come up in a game that I was running or playing. Still, it could only be a good thing for the PCs and the campaign, as you as GM get to adjudicate exactly what consequences the PCs' one-use undo button will have. The only way it could go wrong is if you decide to go all malevolent genie on them, and give their well-meaning actions horrible consequences - but if you're the kind of GM to do that, the PCs are probably doomed anyway; drawing this card will just make them be doomed faster.
For the PCs: Benign.
For the campaign: Benign.
The Flames
The PC gets a new NPC enemy who wants to kill them. Given that nearly every D&D adventure already involves NPC enemies trying to kill the PCs, one more won't make much difference.
For the PCs: Benign.
For the campaign: Benign.
The Fool
Losing experience points is annoying, but it's ultimately just a temporary setback, especially as it can't reduce the PC's level. (That being said, I usually run sandbox hexcrawl campaigns in which the players have a great deal of power to decide what level of challenge they face, so PCs having different levels in the same party isn't much of an issue for me. Your milage may vary.)
For the PCs: Unpleasant.
For the campaign: Benign.
The Gem
There's not much that can go wrong with free money. Fifth edition doesn't require the PCs to have specific levels of wealth in order to be effective as adventurers, so a sudden windfall or loss doesn't make much difference.
For the PCs: Benign.
For the campaign: Benign.
The Idiot
Permanent intelligence loss is a frustrating inconvenience to a character whose build depends on having high intelligence, and not enjoyable even for PCs who doesn't especially need their intelligence for anything. It's unlikely to harm the campaign as a whole, though.
For the PCs: Unpleasant or awful.
For the campaign: Benign.
The Jester
Bonus experience is unlikely to cause problems, except maybe envy. (That being said, I usually run sandbox hexcrawl campaigns in which the players can influence what level of challenge they face, so PCs having different levels in the same party isn't much of an issue for me. Your mileage may vary if your campaign gives players less power to decide their challenges.)
For the PCs: Benign.
For the campaign: Benign.
The Key
Given that you're in total control of what magic weapon the PCs get from this card, this can only go wrong if you deliberately make it go wrong.
For the PCs: Benign.
For the campaign: Benign.
The Knight
In my experience, players love getting a free and perfectly loyal minion, and think it's awesome. For you, it's an NPC with a mysterious past who was created by the card or transported to the players by magic, and those're both interesting possibilities to explore.
For the PCs: Benign.
For the campaign: Benign.
The Moon
Speaking as a GM, wishes are tremendous fun to hand out to players. Since you're the one who decides how to grant them, you're in total control of whether they cause problems for the PCs and campaign. I imagine you don't want to wreck the campaign; whether you want to ruin the lives of your PCs is a very personal question I won't ask you to answer.
For the PCs: Benign to devastating.
For the campaign: Benign.
The Rogue
The PC gets a new NPC enemy, with the twist that it's someone they know already. As with The Flames, PCs already expect to be up to their armpits in enemies, so one more won't mean much. Given that you get to choose which NPC betrays the PC, this is unlikely to cause major problems unless you pick an NPC the PCs really like or need, like the Paladin's god or something.
For the PCs: Unpleasant.
For the campaign: Benign.
Ruin
PCs hate losing money, but it's not actually that big a deal. PCs can't buy magic items in fifth edition, so this really just means they'll need some other party member to cover their food and lodging costs until the party next finds some loot.
For the PCs: Unpleasant.
For the campaign: Benign.
The Skull
The PC either dies forever or doesn't, depending on how well the fight goes. If the other PCs are foolish enough to jump in and fight too, this could lead to a TPK - but if they either go all-in and work together or let the PC who drew the card fight alone, the risk is minimal.
In my campaigns, I generally don't consider a permanent PC death or retirement to be a big deal, and so rarely have problems throwing such threats at my PCs - but I understand your group values PC wellbeing more highly. You might want to skip or nerf this one.
For the PCs: Either benign or devastating, nothing in-between.
For the campaign: Probably benign, but potentially devastating if the other PCs decide to help the one who drew the card.
The Star
Free stats. Everyone cheers. In my experience, this won't be disruptive; as nice as a +2 stat increase is, it's small and non-impactful enough that I and my players could both completely forget it ever happened within a few sessions.
For the PCs: Benign.
For the campaign: Benign.
The Sun.
Once again, there's not much that can go wrong with free stuff.
For the PCs: Benign.
For the campaign: Benign.
The Talons
The PC loses all their magic items. Many classes and builds need gear to be useful, but they don't specifically need magic gear to be useful - so, while this does suck for the affected PC, it's unlikely to be fatal unless it results in the PC being unarmed and unarmoured in the middle of a dungeon or something.
For the PCs: Unpleasant or awful.
For the campaign: Benign.
The Throne
The PCs get a free skill proficiency which applies in social encounters, which is nice. They also get a sidequest, for which the reward is a cleared-out dungeon... Which is technically the same reward they get for a lot of quests, if you think about it. Neither of these rewards is likely to cause problems.
For the PCs: Benign.
For the campaign: Benign.
The Vizier
The PC gets a just-tell-us-the-puzzle-solution-or-knowledge-check-result-already voucher, valid for one in-game year. Since you're running a sandbox campaign, this won't cause any problems that last longer than an adventure - and you can definitely trust me on this. I like to give my players access to an oracle that can answer nearly any question about half-way through each of my campaigns; it's never caused a problem yet.
For the PCs: Benign.
For the campaign: Benign.
The Void
This is basically the Donjon with a different coat of paint.
If the party already has access to a wish spell, or you're willing to fiat them some means of finding their comrade's imprisoned soul, this is just a spin on the classic "go on a quest to resurrect/rescue the dead/captured party member" adventure, and the traditional ways of handling that scenario will work; otherwise, the lost PC has essentially been forcibly retired.
For the PCs: Awful or devastating.
For the campaign: Benign.