I have yet to get my tank permanently stuck but it can take a very long time to get unstuck in rare circumstances. I once got stuck to a tree and the unstick code slowly slide me out but my tank was at a steep angle and I continued to slide stuck in the ground for something like 5 or 10 minutes until it finally popped out of the ground and I could drive away (and be immediately killed by an enemy who, during all that time, finally found me). So it requires some patience.. sometimes quite a bit of patience.