The kubel is simply broken. It compeletely slaugthers USF early game, and makes it impossible to fight in the mid-late game.
One way to fix it is to put the WC 51 at 0 CPs, that way atleast USF can go mechanized company and kill the kubel.
USF might need more units in T1 or a tier restructure if the kubel is to keep it's role of a T0 mobile suppresion unit, however.
0 CP WC 51 isn't a good idea because in 1 vs 1 the Wehrmacht players will have much fun and it will change the USF to "go mechanized company or die".
In my opinion the Kübelwagen should be like the Motorrad in vcoh: a non-supression unit which is a cheap scout that supports MGs.