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
Whether this card is disruptive or not depends entirely on how your group handles alignment and forced alignment changes. 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 are 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.
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 embarassing 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; otherwise, the lost PC has essentially been forcibly retired.
For the PCs: Awful or devastating.
For the campaign: Benign.
Euryale
Slightly worsened saving throws is a minor inconvenience at worst.
For the PCs: Annoying.
For the campaign: Benign.
The Fates
This is a good thing for the PCs, unless 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 their inevitable doom happen 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 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.
For the PCs: Annoying.
For the campaign: Benign.
The Gem
There's not much that can go wrong with free money.
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: Annoying or awful.
For the campaign: Benign.
The Jester
Bonus experience is unlikely to cause problems, except maybe envy.
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
For the players, this is a free and perfectly loyal minion, and therefore awesome. For you, it's a warrior with a mysterious past who was created by the card or transported to the players by magic, and those're an interesting possibilities to explore.
For the PCs: Benign.
For the campaign: Benign.
The Moon
Since you're the one who gets to grant wishes, you're in total control of whether this causes 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: Annoying.
For the campaign: Benign.
Ruin
PCs hate going losing money, but it's not actually that big a deal. PCs can't buy magic items in 5e, 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: Annoying.
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.
For the PCs: Either benign or devastating, nothing inbetween.
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.
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. This sucks, but is 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: Annoying 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.
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.
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; otherwise, the lost PC has essentially been forcibly retired.
For the PCs: Awful or devastating.
For the campaign: Benign.