I'm not quite sure if I'd put stuff like Fiamma's 40km sword or Accelerator's Black Wings at "city level", although it dealing with Magic God stuff certainly isn't due to a specific type-matchup of some kind against them. It's just that strong for one-and-done effects. Coronzon's barrier very clearly is not made by a Magic God (she isn't one and she fuels the barrier with her energy iirc) Though I could be missing something regarding it's failure against lower tier attacks.Imagine Breaker hits its limit around City level (unless its dealing specifically with the Magic Gods or something iirc?) and because FGO has no brakes the current baseline for an "Average" Servant is like......continental or something now.
Thing is none of the FSN cast besides True Assassin are "Average". Heracles and Arturia both have planetary feats to their names, Gilgamesh goes even higher than that, Medusa and Cu have both tangled with extremely heavy hitters and can stand even with Herc and Arturia, Medea's younger and weaker self made a literal pancake out of a being that can levitate a landmass the size of a small country, and Koujiro.....would require a ten paragraph long explanation of Nasuverse sword autism so take my word for it when I say he could probably cut through IB's erasure ability itself.
Emiya is closer to average but is obscenely versatile, has combat precog that let's him keep pace with a guy who can blitz him, and can punch out of his weight class because of even more Nasuverse specific mechanics that equate to "he can make his swords into suicide bombs"
Not sure about that Kojiro part but I'll take your word for it.