Q: Who would be stronger if Servants fought the 27 Ancestors? Servant's also have superhuman battle ability, but the 27 Ancestors are also superhuman monsters.... I got the impression that ORT, Primate Murder, Altrouge Brunestud, and so on were obviously stronger than Servants.
A: All I can is they're both powerful.... They're both beings that live in mystery, so the only thing that can be said is that it all depends on the situation the moment it comes done to ""having the means of killing the other guy somehow". Among those, Earth rules won't even apply to ORT in the first place and Primate Murder has the super advantage against primates so they're in a league of their own, I suppose.

Fate/complete material III: World material - FAQ with Nasu: Miscellaneous