From skimming the lower scored reviews, looks like some of the main criticisms are "it's a lot like the previous games so it deserves a lower score" and "it didn't get political enough with the story."
Slant Magazine -- "Divorce it from its narrative and Far Cry 5‘s biggest overall problem is just how little mechanical innovation separates the game from Far Cry 4."
Fandom -- and, of course, the inevitable, "a game that adds nothing of substance to either genre or franchise."