I am very much not a rocket scientist. But off the top of my head: Cooling. In space there's no ambient cooling by air. If you want to cool something, you have

I am very much not a rocket scientist. But off the top of my head: Cooling. In space there's no ambient cooling by air. If you want to cool something, you have to pump heat out of it and into a radiator. And since that is energy-intensive, you want to minimise heat production as far as you can. Volatiles. In a vacuum those will boil off and not come back. All sorts of polymers, possibly including those used in seals and bearings, suffer from this. Electrostatic build up. Again, there's no route for this to escape into the air, so you need to make sure that anywhere it can build up is grounded, i suppose. Maintenance. Depending on where you're going to use it, you might not be able to depend on some guy with a socket set and ungloved fingers being able to fiddle with it whenever necessary. Gravity. I don't know much about hydraulics, but all sorts of machinery is designed around an unstated assumption that liquid will drip downwards given a chance (sumps etc). Some of these problems go away on the moon (which has gravity, and potentially pressurised garages for maintenance) or Mars (which has the above, and also some atmosphere).