Its the cheap price which make it op. 300mp for a bunker with 2 mortar which has more range than axis Counterparts? Hell...and than u dont need mp to recrew it etc. When you get mortar...your Enemy Will lose much mp to recrewing and when he qas succesfully to destroy it...hell...its only 300mp. This is really cheap for its Performance.
You do understand the concept that immobile unit should perform better then mobile unit, because it sacrifices literally 100% of mobility?
You also do understand that 2 mortars/isgs hardcounter it without much of a problem as long as you don't put them literally on top of each other?