Cost, simplicity and contamination tolerance. A gear motor is far cheaper per unit of displacement, has almost nothing to go wrong, and survives dirty fluid that would destroy a piston motor. On mobile and agricultural machinery those three things often matter more than efficiency.

Considerably better than any piston type, because the clearances are larger and there are no slippers, valve plates or control mechanisms for particles to score or jam. That is why they persist on equipment where oil cleanliness cannot be guaranteed in practice.

Internal leakage past the gears is higher than in a piston motor, and it becomes a large fraction of total flow at low speed and low pressure - so efficiency falls off noticeably in those conditions. At moderate speed and pressure the difference is smaller but still real.

No - displacement is fixed by the gear geometry. Speed therefore follows the flow supplied, so any speed control has to come from the pump, a flow control valve or a variable drive. That is a system design consequence rather than a limitation of the motor alone.

More so than piston or vane motors, because gear meshing generates both pressure ripple and mechanical noise. On mobile equipment that is usually acceptable; in a workshop, building or any noise-sensitive installation it can be the reason a quieter type is specified instead.

Fan and pump drives, conveyors, agricultural implement drives, sweepers, and auxiliary functions on mobile plant generally. The pattern is moderate speed and torque in an environment that would be hard on a precision motor, where cost and robustness outweigh efficiency and where a failure means a breakdown in a field rather than a call to the maintenance department.

Wear of the gears and the housing bore from contamination, which shows as lost speed and torque as internal leakage rises, and shaft seal failure from excess case or return line pressure. Both are gradual, so performance degrades before anything obviously breaks.