The sheer hypocrisy from OKW players in particular on this is astounding. Every single one of these complaints and worse applies to the ISG. It has comparable range, is mobile, has incredible accuracy, is much harder to hit thanks to its small size and is practically immortal since it will be sitting next to a battlegroup HQ and eventually flak HQ. If by some stroke of luck you stand in the fire from a mortar pit long enough for it to get lucky and kill the ISG, you can just re-crew it.
I agree that it has an insanely high survivability, but when the mortars and ISG are in enough range for each other the AOE lets the mortar win (mostly from my own experience) Is it a problem, surely not, since the ISG has so much other outstanding qualities.