This is the crab module we used on our drive this year. Using servos for module rotation worked out very well with the low friction field. There was no need for any feedback and we had plenty of torque.
The gears are mcmaster, with metal servos horns attached.
We also used mechanical slip clutches (torque limiters) on the drive. Control your max torque, and you have a very simple and reliable traction control.
Out of our 67 matches this season, we only had to replace four servos, and a bb-motor.
We had a great season this year, and have a new respect for Hitec servos.