Fate/Zero

[Ryllharu]

Not getting killed by your own Servant right off the bat is important too. 5th War Caster, who had a master who was never even given a name.

Raw power doesn't matter when you're not wary enough to control your own servant, or when you summon one that is too strong for you, or has conflicting ideals. Which is why Kiritsugu expressed concern about summoning "Arthur" given his chosen methods of combat.

Wouldn't be much use if you were plenty strong to summon a powerful hero, but had to blow through Command spells to get them to listen. At least some of the mages are thinking about this in the 4th War. Even Rin didn't think about who she could get, she only thought about getting someone strong.