I apologize, I consider myself somewhat of an expert on this particular universe, so I'm unintentionally applying far too high standards on a Free roleplay, especially one that I'm not even GMing (not trying to backseat GM, sorry~!)

The main problem I have with these "OC" characters is that they don't follow the standard formula for becoming a Servant in the universe. Basically, Servants must have been some variety of legendary hero in their past life, who transcended death by having their spirit summoned by the Holy Grail. There are some extremely anomalous exceptions, such as Fate/stay night's Archer, who was summoned from the future due to a peculiar paradox regarding an amulet that he coincidentally possessed both in the present and in the future. 

Because the F/SN universe operates with a "Multiple universes" theory, the excuse that it's an alternate universe isn't exactly as solid, although it's not impossible for a GM to just mess with it as he pleases.

Overall, I have no intention of leaving quite yet, and I also don't really want to waste my time lecturing people on something that the GM seems to have no problem with.