It's absolutely huge in a party that can set up combos when they know a crit is coming, like a consistent 238 average damage with a grave cleric and a high-level paladin's first attack (or mid-level paladin/sorc multi-class, or hexblade with eldritch smite). Or if have another way to impose vulnerability for one attack.
It would also be quite good for a high-level rogue with lots of Sneak Attack dice.
If you have paladin levels for divine smite, you can dump a high-level slot whenever you do eventually crit. But that might not be until late in a battle (and taking some enemies off the field early means they had fewer turns to damage the PCs or whatever). Or you might not crit at all during a fight, so you might use those spell slots early on non-crits.
But some abilities have to be used ahead of an attack, so can't be used in reaction to rolling a crit. If you know when you will crit, you can stack those up:
- paladin Banishing Smite (5th): the attack deals an extra 5d10 force damage to the target. - it's part of the attack so it's doubled by crits
- cleric Path to the Grave (grave cleric Channel Divinity) - vulnerability to all damage from the next attack that hits.
Grave cleric readies an action to channel divinity as the paladin or hexblade is moving in to attack. Paladin casts Banishing Smite, then attacks with an auto-crit and does a 4th-level Divine or Eldritch
Smite (the max 5d8) on top of that. Lets say their base damage is 2d6+5.
On a non-crit with that setup, against non-undead, they'll do 5d8(divine smite) + 5d10 (banishing smite) + 2d6 (weapon) + 5 (str), for an average of 62.
On a crit, average of 119.
With Path to the Grave vulnerability, double that for 238 damage average (max 418) in the first round from their first attack, absolutely guaranteed no way to avoid unless they're resistant to some of the damage types, or if they have Mirror Image or Armor of Hexes up to make the attack miss regardless of the d20 roll.
Also, if that attack leaves the target with less than 50 HP, it's banished (no save). (The wording seems different from Banishment (4th) - if not native to this plane, it's banished to its home plane, full stop. You don't have to maintain concentration for the full minute to stop it from coming back; that part only seems to apply to banishing a native creature to a harmless demiplane.)
So if a creature is banished, then the whole party can heal up and ready a fresh round of attacks.
With more PCs getting in on the action,
fighter Commander's Strike: let another creature make an attack, adding your superiority die to the attack's damage roll. So maybe the paladin casts Banishing Smite at the end of their first turn, then after the cleric uses PttG, a battlemaster fighter can use a reaction to have the paladin attack with an extra 1d8 damage that gets doubled on crit.
Valor Bard - Combat Inspiration lets you add it to a damage roll. Or the optional Tasha's rule Magical Inspiration lets any bard's inspiration die be added to a damage roll. (The phrasing of those aren't 100% clear that they're part of the weapon damage roll, which is required for it to get doubled on a crit.)
Holy Weapon (5th) - the cleric could cast Holy Weapon on the paladin's weapon before combat, or on the first turn and do this combo on the 2nd, making attacks with the weapon do an extra 2d8 radiant on hit.
So potentially getting an extra 1d8+1d12 (or whatever size of inspiration die) doubled on crit instead of when they're spent in most other cases. Plus the 2d8 holy weapon on every hit including this one.
Or for a caster, a high-level Inflict Wounds (level+2)d10, e.g. 9d10 for a 7th-level spell slot. 18d10 on crit averages 99 necrotic damage, vs. 75 for a 6th-level Disintegrate of 85.5 for a 7th-level Disintegrate on a failed save. (Nothing on success, that's why Disintegrate (6th) does so much damage.)
At mid-level play, a 3rd to 5th level crit Inflict Wounds is even more serious business compared to regular 3rd to 5th level spells. Like 55 average damage for the 2x 5d10 of a 3rd-level Inflict Wounds, vs. average 28 on a failed save for single-target Fireball (3rd)'s 8d6.