IS have 0 (zero) AT capability unlike rifles, grens or volks. You also need to invest like 300mp and 50+ fuel for all the upgrades to let them fight on an even playing field with axis infantry.
Volks need a vet buff but IS are fine.
What does AT capability has to do with anything? As a whole faction, IS with a decent support REK anything. One Vet 3 LMG IS is a beast. UKF as a whole does not struggle versus armor either. Those AT capabilities which you have mentioned are merely Snares. Except for volks, and Volks blobs are not bug issues anymore. Handling Volks Blobs in 1s and 2s is L2P issue.
If anything, Rifles' buff is more justifiable (excluding Rifle Company) than UKF since the whole faction is powerful. Also, IS -if used right- are even deadly in the beginning of the game, unless you want to YOLO RUSH LMG grens, they are also not that expensive to reinforce anymore. Their vet 2 and 3 are damn good.
Instead of demanding huge buffs for Grens, PzGrens and Volks, why not just tone down IS and Rifles. Although, OKW as a whole has bigger problems.