Pak Howitzer comes much later, is much more expensive, and is meant to be a light howitzer rather than a mortar. The ISG has pretty similar veterancy to all other mortar type units, except it doesn't get extra accuracy for the barrage that other mortars get since the indirect fire balance patch.
I wish pack howi would go all-in on the light howitzer, get rid of auto attack and add however many shells to barrage to make it cost effective.
Its 60 range default shots really make it exclusively defensive weapon compared to 80 range mortars that can actually retreat if threatened.