[Ether]

Originally Posted by EricH

Or if the rollers too tight, or are rubbing hard against the sideplates.

I agree with the above, but will add this: a high-quality mecanum wheel's rollers will not be "too tight" or "rub hard against the sideplates". The rollers will have properly-designed bearings that can withstand the high axial thrust loads and still spin freely with no binding or rubbing. That's what makes them expensive.

This is another of those cases where theory says one thing, and reality says to theory, "You need to come over to MY place tonight!"

There's no reason that theory cannot include a consideration of the effects of friction. In many complex systems, a proper theoretical analysis of frictional effects is absolutely mandatory for developing a proper design.