He is correct, one terrain texture type does affect GVs making it bouncy and slow to climb hills. It looks like a field of maybe 6" to 12" rounded stones. Most terrains make little use of this texture tile, but some use it more, especially on hills.
Also, didn't some versions of the M4 Sherman have gyroscope stabilized guns? OK, Wikipedia says they did but it wasn't quite good enough to fire while moving.