This is my honest opinion after 5 hours of play..
Lets get the bad out of the way first, becaue I really don't think that the AI is that bad.. Oh, and I am playing on extreme difficulty.
But their reaction time is sometimes a little to slow. The should see the ghost within a second or so.
Other than that I haven't had a whole lot of troubles with them.
I have tested some of the scenarios where a lot of people is complaining about them, and this is how it went for me:
Complain: The AI ghost is garbage. I have NOT had a single time where they got me busted or was in my way. But I have found them extremely helpful when in trouble. What I usually do is I park them outside a camp and I infiltrate on my own. If things backfire I call for backup. They might be overpowered, but I don't find them ruining my experience at all.
The AI automatically knows where you are. This I have tested with both the cartel and Unidad. I went to the unidad FOB, attacked it and got reinforcement after me. I wen't into the trees and bushes to hide and changed location a couple of times. I could see Unidad searcing for me, and hearing it over the radio, but after a short while, they called of the search and went back. On their way back, I shot at them again to get their attention. Once again they went after me, and once again I got them of my back by hiding. At no time did they know my exact location, other than where they saw me before.. They even say so over the radio, something like 'There is no enemy at the last location, spread out' or something like that. Same goes for the cartel. Not once have they magically found their way onto me.
The AI don't take cover. I rarely find enemy AI in the open after they have been engaged. They are very good at taking cover, and even though they sometimes take cover making them even easier to kill, it just shows they don't know you are around because they havent seen you.
On extreme they are pretty good at aiming, but never over- or underpowered.
I have also have some cool situations, where I was attacking a little cartel camp, and out of nowhere, a group of rebels comes along to assist, without me doing anything.
So while the AI might not be perfect, and I am sure you guys are not lying about your experience with the AI, I really can't see it being THAT BAD. I find it totally fine.