Unlike the previous games and the ones after it, the covenant in Halo Reach especially the elites are damn smart. They are extremely hard to stick, they actually throw grenades, and will actually advance toward you when you run for cover. The marines and noble team are another story sadly. They are stupid and useless even on easy difficulty.
Needless to say, the ai in Halo 4 is awful, and Guardians has the worst ai I have ever known in a video game. Just watch The Act Man’s latest Youtube video as of today when he discusses about the god awful laughable ai. 343 has ruined the excellent enemy ai in Halo Reach by downgrading their intelligence and making them meaninglesss bullet sponges(like the prometheans for instance).
Hopefully 343 can improve the ai in Halo 6 and if they fail to do so, I hope another company takes over like Creative Assembly for instance and 343 goes bankrupt because they have ruined Halo with 99 problems and the ai is one.