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."
I was honestly shocked to see so many critics say that they didn't change much in Far Cry 5, leading to it feeling stale. From what I've seen they've made alot of changes, too many changes imo:
-They dropped radio towers for unlocking the map. While I get why they did it, because people have been bitching about the Ubisoft tower mechanic for years, I personally thought Far Cry did the tower mechanic better than any other Ubisoft series, each tower was basically a little mini platforming puzzle, and you got free weapons unlocks for completing the towers.
-They dropped the experience based skill system, in favor of a challenge based perk system. I personally don't like this, because the new system forces you into trying multiple weapons and play-styles in order to complete all challenges and unlock every perk. Historically Far Cry was a play your own way style game, you could complete the whole game with stealth, or run and gun the whole game, you could use just stealthy silent weapons like bows and silenced sniper rifles, or go crazy with flamethrowers and shotguns. Instead of letting you play the way you want to, the new system forces you to try every playstyle and weapon, not a fan.
-There is no longer a reward for clearing an outpost stealthily afaik, big disappointment for me there.
-You no longer use skins to craft pouches and weapon slings and such, another big disappointment for me there because it makes hunting feel pointless.
-There is no longer a structured story, you can complete the 3 regions in any order you want to, clearing them each in succession, or making progress on all 3 as you go. Need to actually play it to comment on rather or not this is a positive change.