1. Define "hardcounter". Exactly how many ISG shots kill a mortar pit? How many ISGs do you need to kill a mortar pit before it inflicts some serious damage to your army? Now, the same question while mortar pit is using "brace" ability.
Its completely irrelevant, it could be one shot, it could be 100 shots, as long as mortar pit can't shoot back at ISG, its hardcountered by it, there really isn't much to discuss on this matter.
2. Your question is the story of axis life in COH2, extended to many units.
All that ZiS-3 AT superiority.
That soviet stock mortar being superior to ost one.
That T34 superiority over P4.
And so on and so forth.
Double standards is awesome thing to have!