Some genres have an easier time getting high review scores. Games like FPS appeal to more people, and games like JRPGs appeal to fewer people. 81 is kinda low for a shooting game, but pretty high for a JRPG.
Anyway, any game that gets 80 or above on Opencritic/Metacritic is a worthwhile purchase in my book. So long as said game doesn't include shady pricing like MTS, or on disc DLC.
I bet Far Cry 5 is getting a slightly lower score because of franchise fatigue. Some reviewers are just tired of the same formula being followed. It happens to a lot of games. Poke'mon started to suffer from it until X/Y reinvigorated the series.