It is a fucking truck guys, wha the hell do you expect? If anything the Hanomag is under-armoured.
And the Katyusha is a tricky weapon to accurately portray in an RTS with the scale of CoH. There were used en masse, not piece-meal, the effectiveness of the Katyusha was never down to the individual units, but their numbers, using the rationale of "If you throw enough spagetti at a wall, some of it will stick" they were horribly inaccurate. So as it is, let's not make any drastic changes.