Poor enemy AI is a legitimate criticism. There are rare moments when Elites don't seek cover, but just let you blast them with 2-3 sniper shots. Also, enemies can't fly banshees for shit. It's completely common at the end of a fight for one or more Banshees to be stuck flying into a corner, still being piloted but unable to move.
Friendly AI is sometimes just as bad, though. Warthogs in single player are often worthless, since the marines can't drive for shit and they won't make any effort to hop in your gunner seat unless you can pull up within 10 feet of them. This isn't always possible.
Still, TEH-CJ's complaints are terribly reductive and not altogether accurate. While the AI is occasionally bad, it's still as good as any other Halo game 95% of the time. Calling it "linear on rails" is just fucking bullshit. The level design is the same as it's always been - a string of open arenas separated by entrances and exits. It might feel less open because doors often close behind you, but that's just to prevent backtracking and player confusion. I have no idea from where he got his criticism of death animations and blood effects. The Promethian Knights are totally enjoyable to fight, and their death animations are explained very well within the fiction (as noted by Cortana shortly after seeing the first one die).
The campaign is a total blast. It's my favorite campaign since Halo. I've played it multiple times and my biggest criticism is that I just wish there was more of it.