Someone who asks for R/M/E only is a moron, hands-down. Precisely for the reasons of OAT GS and Kantonotachi as you mentioned, but also OAT polearm for DRG (and if they endorse empy polearm, just blacklist them forever).
If they don't accept you with OAT GS and a proper hitbuild, they're the sort of leader whose group wipes to Qilin. True story.
So if they don't invite you because you're not using R/M/E on DRK, SAM, or DRG... You're probably better off not wasting your time with people using such a braindead metric or even demanding inferior weapons.