Are we talking about can't be retreated ever or just temporarily? I've been hit by them and the temporary time was enough for me to lose all my men. The last couple guys get killed on retreat (something axis does better than allies due to higher accuracy values of troops).
I can't speak for other people, but in my case my units were completely unable to move or return fire for ~15 seconds after being hit by the stun grenade. My units were wiped before I was able to do anything. I couldn't say if the effect is permanent, but it's definitely much longer than intended.