Its inner rotor has one fewer tooth than the outer ring, so the rotor orbits as it rotates and the output shaft turns much slower than the displaced fluid volume implies. That built-in reduction is what a gearbox would otherwise supply, and removing the gearbox is the format's main advantage.

A geroler places rollers between the inner rotor and the outer ring, so contact rolls instead of sliding. That raises mechanical efficiency and starting torque and extends life at pressure. It is worth the extra cost on any duty that has to start under load rather than accelerating freely.

Wheel and track drives on mobile machinery, augers, conveyors, winches, sweeper brushes, and agricultural and construction attachments generally. The pattern is a low-speed rotating load in a position where a gearbox would be expensive to fit, awkward to service and one more thing to leak - so driving the load directly from the motor shaft is worth the efficiency given up.

Speed and efficiency. They do not run fast, and their overall efficiency is below a piston motor's, so duties needing high shaft speed or the best energy performance belong to another type. Their strength is torque density and simplicity at low speed, not efficiency.

Because side load will destroy a motor not built to take it. Wheel-mount versions have integral bearings sized to carry the wheel load directly; a plain shaft-mount motor used in that position will fail at the bearing quickly. Match the mount to whether the load is radial or purely torsional.

Yes, and reversing is normal for wheel and winch drives. What needs checking is the case drain and shaft seal arrangement, since some designs are sensitive to case pressure in one direction, and a blown shaft seal from excess case pressure is a common and avoidable failure.

Contamination and excess case pressure, in that order. Grit wears the gerotor element and opens clearances, which shows as lost torque and speed; case pressure above the seal's rating blows the shaft seal. Both are system problems rather than motor faults, and both recur if only the motor is replaced.