The AI are not overpowered at all, put it in context. Why should the human players be the only ones capable of head shots? It's designed to create fear of death (not literally)
. Meaning, if you see 4 AI bots, you don't immediately think "right I can take em". If your learning anything from the game, it makes you approach differently. You aren't special, you are a team member. Your guns are not frag magnets, your abilities aren't pivotal enough to lord it over other players. AI is no more capable of repeated head shots than say a well practiced fps player, which is the point. You have to earn things in this game.
Also the computer AI is not stupid, but they do respond to your actions more than an all autonomous team. Wherever you go, they change tactics to try and support or help, which might sometimes have them all splitting up. Alot of the complaints come from people who seem to have designs on a static class and load out too. If anything the single player campaign gives you a friendly nudge into trying all aspects of the game before you settle on a given class or two classes, so any team fortress comparisons are rendered moot. Sticking with a class should be reserved for an all human game really.
And yes, the campaign, while not a breeze, is fairly easy once you adapt and support instead of being chuck norris. There are some lag issues online, but in essence, this game experience is awesome. There's nothing quite like slide tackling some unsuspecting guy then unloading into him with a kross SMG before making an escape via wall jumping up to a ledge.
My advice is just to try it how it was meant to be played. I know it's not as glamorous as smoking every living thing in sight, but try supporting. If you see an opportunity to kill, take it, but don't leave your team exposed for the sake of glory. Even if you've tried that and the game isn't for you...there will be limited sympathy when you spam forums saying "this game lied to meeeee"