Main advantage. You can achive a great gearratio and humongous torque in a rather limited space from such a drive.


Disadvantages.
Very inefficient. Up to 60% of the input power can get lost. That is why excavators and such have huge oiltanks and radiators on the oil circuit. Not a big deal on those machines, with their diesel engines.
Heavy, there was no big reason for the manufactorers to build the stuff light. That is changing , but it means it's getting even more expensive.
Complicated and messy. 1 oil leak, and the inside of the robot/arena/workshop becomes a skate ring.

If you want it as a simple solution for combustion power conversion, remember most venues won't allow combustion engines.