I've found that sometimes tanks, and particularly IFVs armed with missiles, tend to blow themselves up when firing up hills.
It's not always possible to tell from the map when this is going to happen so you have to set things up and watch in the mission. If you have a group of tanks heading towards enemies on the other side of a hill, it's best to give them a waypoint with no action first and only give the 'attack enemies along path' when they reach the top of the hill.
There is a maximum number of moving vehicles you can have in a mission. This is about 200. I don't think it matters how many groups you have. One reason to use groups rather than individual vehicles, though, is that there is a limitation on how many groups JSTARS can track.
Using helicopters and attack aircraft on CAS can also make the outcome of ground battles more unpredictable. I have found that helicopters tend to be more effective when told to attack a specific target, so I tend to give them two waypoints over the battlefield: one for the specific target and then a CAS action.
It's tricky to get CAS to work properly for AI aircraft but when it works, it is very effective. The AIs tend to do best when given a heavy bomb load and the radius of action is small with no buildings nearby.