To say that a stealer vs the assault cannon is heavily in favor of the assault cannon is an understatement. With 3 dice and killing on a 5+ (4+ with sustained fire), an assault cannon will drop the stealer 70.3% of the time(87.5% with sustained fire). Since the kill rate is already pretty good, the assault cannon can be used on the move unless you just have to kill that target this turn. Ofcourse it has limited ammo and can explode after being reloaded but its explosion rate is 2.8% for those last 10 shots so you can expect 1 explosion every 4 games or so.
Overwatch is a nightmare for stealers versus the assault cannon. Even with the reloaded gun that can explode, it looks unlikely for any stealer to get beyond the initial square if the cannon has not exploded.
The numbers on the plot are the probabilities of the outcome for a stealer trying to go 3 paces at which point the results have pretty much leveled out. The stealer is dead but maybe the gun has exploded. This is a log plot compared to the earlier plot where I have cut off the stealer being alive without the gun exploding to keep the scale reasonable. Chances of a live termi and live stealer after moving 6 squares in overwatch range is 0.0007% or about 1 in 100K. Not very good.
Why would you then charge an assault cannon on overwatch. First is that he has to shoot. He has no choice so he uses up 1 precious ammo counter. Also if he is low on ammo reloading should take him off overwatch which means to reload and return to overwatch will cost him 6 command points during your turn which is more than what he probably has.
An interesting thought for stealers is that closing a door infront of the assault cannon on overwatch causes him to shoot the door. He will probably open back up for you so while you did not go a square forward you did cost him an ammo and moved him one step closer to having to reload.
Next Up is the series is shooting the Broodlord.