Should I grail Sakata Kintoki to 100?

Kintoki becomes a bad berserker in the future :'v

As this event showed, Kintoki in not fitted for hard challenges. He is great at nuking a boss, especially a NP-B-B Brave chain but like you said, he may die too quickly with a stray crticial or if he is focused on.

Generaly, you grail who you want, it is more a matter of likeness than a gameplay-wise.

Just grail who you liike. If we're talking gameplay it probably best to grail your 4 star as they will apreciate the boost more than your kintoki who's already strong without it.

Grails doesn't make him any sturdier, he is cursed with bad survivability, that's why he falls off to contenders like Herc or Cu Alter, even fully raised Hyde would have better survivability than him.

Just use tanks to cover this weakness of him, he should be gucci with tanks. Still bad choice for enemies who have AoE attacks like big ghosts or dragons though.

As a bond-10-Kintoki-user, my answer is 'no'. He is strong indeed, but wasting 5 grails on him will not make him much better.
Kintoki are just "trump card" servant. You give him a 50% charge CE, activate first and second skills, and do instantly ~100k of universal dmg from np to anybody or like 200-250k dmg from chain... and that's it. There's nothing else he could offer. 3rd skill can heal back like half of one enemy attack health (but usually Kintoki just dies from one good crit).
So the point is: grailing him will increase that 100k to 110k and nothing more.