The awkwardness comes either in their timing or their performance. They aren't mortars, but they aren't artillery like the LeFH or any of the rocket artillery units. Trying to place them somewhere in that middleground is the awkward part.
I personally would find ISGs performing as mobile artillery pieces, instead of glorified mortar teams with a different model, to work better. But my point is that they're awkward trying to be both, but acting as neither.
Ya mean they should cost a lot more and perform a lot more? Probably can't for OKW, since ISG is too alone in OKW's unit list (making a higher cost more painful to wait for if they really need it) and arrives too early for that (ironically the opposite reason as well, since the artillery units can't be built off the bat for reason eh?). The concept could be possible for Pack Howi since now USF has a mortar to use, but turning it into that would probably Motor Carriage even less used.