Can someone give me a rundown of why Claw is so bad on BK?

So first thing first, claw bk isn’t bad. But there are reasons sword is more popular, the TL;DR is that it’s simply easier for swords bks to make high dmg and have decent durability.

From a stat build perspective, bks need crt and a bit of dex. A sword bk can fulfill the stat requirements quite easily, just add str and vit but a claw bk would need to add agi to be effective. The main reason to even use a claw bk is for massive absolute/magic evasion, and 256 agi is a required minimum for an evasion build. So a sword bk has 4 stats to worry about whereas a claw bk will have to balance 5. Generally speaking this means that you will have to make a cut either with your hp or dmg, neither are really ideal.

On top of that, the nature of evasion is pretty counterintuitive to a tank. Technically speaking, claw bk does have the higher defensive bonus, but also technically speaking, it can not matter. You can get hit 9 times in a row and die even with the bonus. Comparatively to a sword bk, you start with a base 60% melee r (guard 2+alleviation). And that will ALWAYS work.

Auto attack is also not the most op on bk, because one of their main focus is double attack (bite). You can’t use autoskills like veleno when double attack procs. Makes it kind of not worth it.

All that said... I honestly think claw bks are amongst the top tiers of tanking in the game. Let me take you down the rabbit hole. Back when I played a bk I was able to achieve 80% absolute eva and 80% melee r. This translates to you taking 20% of the total dmg 20% of the time. At 95% res you take 5% of the total dmg 100% of the time. Over 10 hits the 95% res would take 50% dmg (5%x10 hits), but the 80%abs/res would take 40% dmg (20%x2 hits). Beyond that, there is the eva stat that would make you evade even more, so you’ll probably evade 85% or more of all attacks. On top of that bk can inflict fear which improves the effectiveness of the eva stat bringing the total beyond 90% total evasion or more. Finally, the cherry on top... they can make a clone, so you honestly probably will never take direct dmg if you don’t want to.

If you want to make a nice claw bk, it’s very possible and rewarding, I’d highly recommend it, as long as you remember that first and foremost this is a beast KNIGHT and should be treated as such.

Claw - chance to evade by X
Sword - always resist by X

Well, The new fear howl adjustment can now buff the party's dmg, but it requires the enemy to have fear so against bosses with High resist its PIE.