Vehicles of any sort are not ever going to be immune to a stability check unless they are dug in or something. Impacts, especially from kinetic weapons, easily knock them out of control. Heck, a vehicle can actually flip if the right conditions are met simply crossing the center line of a two lane paved road. (At least that's what several road design engineers teaching my classes have stated-I've not personally seen it happen...yet.) This really boils down to being a rule of common sense. If conditions are such where it makes sense the vehicle is braced against incoming fire (such as bunkered in or having outriggers deployed like a fire truck or Terran Seige Tank) then ignore the stability roll if everyone agrees. If the same vehicle is in the open, moving, and/or on unstable terrain...it may be even easier to send spinning off. And yes...tanks can be knocked cattywampus when hit. THIS I have personally seen. A 120mm round carries a lot of kinetic energy.

Ground vehicles are really only at an advantage on a prepared road in terms of speed. Even on open ground. Having ridden in a few USMC Amtracks across various terrain, they are much slower than what a mech could be for a variety of reasons all related to how the terrain is traversed and that any troops in being carried are subject to SEVERE injury the faster the vehicle goes of diverse terrain. (felt like a loose gumball in a violently shaken tin can and the bus was only going 35-45 mph in the equivalent of a 4x4 mud track.

Short answer to all of this is: I based much of the game mechanics on real world models of combat, physics, and engineering principles as I wrote the rules; and being several decades in the civil engineering field I do have a pretty good insight on such things.