[EricH]

Originally Posted by Ether

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.

Which is probably 50% of the reason they aren't found on FRC robots.

The other 50% is that very few teams, if any, know they exist.