There's a whole section on the interest check that had this. Grand Servants, actual gods, Servants that instant kill, Servants that permanently disable, Character who abuse Imperial Privilege, Characters with overwhelmingly strong reality marbles, and characters with ridiculously high statistics, and or skills. Too name names: Romulus, Gilgamesh, Merlin, King Hassan, Richard The Lion Heart, Artoria, Perseus, Ozymandias, Iskander, Kiyohime (Dragons), Sigurd, Siegfried, Paracelsus, Gawain, Nero, Heracles, etc.

The Fate universe is unbalanced in structure because the authors can usually do what they want. I cannot in this setting. Players are controlling the masters, and characters. Therefore I am retroactively making sure characters are moderately balanced, nerfed slightly to allow them to be used, and making sure players cannot abuse a loop hole ruining the experience for the other players. This is a base line grail war. Meant to be a straight up brawl with trickery mixed in. This is not the roleplay for giants like Gilgamesh to be fighting Heracles, or Romulus.

Servants can be strong, or cunning without being overtly broken. Case in point Arash, Okita, Diarmuid, Leonidas, Mordred, Boudica, etc.

Other servants are on a case by case basis. There is a servant who is meant to be ranked highly on the enemy side but that is because he is to serve as the 'big boss' for the enemy team. However even he is nerfed in comparison to what he could be. The point is to not let fate shenanigans bullshit interfere with a story being told among people who aren't on the same page. I can't have Cu Chulainn chucking Gaebolg half way across the forest, and instantly murdering enemy rider before they can even defend themselves. And yes he can do that. Artoria blocked it, and then got time screwed back into a hit of which she only survived because she literally rolled a natural 20 with her luck stat. Artoria herself isn't allowed not because of her overwhelming power but because she has an incredibly strong base kit, and possible healing from avalon. She even saw Kojiro's phantasm, and herself dying just a few seconds before it hit her because of instinct.