So I've played through the missions 4 times, once per class and tried it again on Hard Mode with my Hunter. I've done some exploration and that was cool.
It's the AI though. They just aren't smart enough. Like, if you're on a mission they have certain rooms that they won't chase you past. The only ones that swarm you are those floating machines, husks, and dregs. They spent too much time behind cover when they had the numerical advantage where they could have just cornered me.
Second problem, and this is something that I loathe about every game that's done this since the 90's, Hard mode just increases the number and HP of the enemies. The AI is still as dumb as they were on Normal mode only now you run into an issue of having enough ammo. But I don't need to worry about that because I can just hide around a corner and wait for my abilities to recharge.
Really like the game so far, though it is more like a demo than a beta. We're getting a preview of the game play rather than testing things out and trying to break them.
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For those claiming Halo had better AI and that enemies always chased you, not entirely true. Even the covenant had their leashes and wouldn't go past certain points. Some of the Flood is about the only thing that didn't have a leash.