For the hydraulic's there was a proposition put forward to increase the pressure to 10,000 psi.
The discussion about it was very reasonable and the outcome was that hydraulic compionents are available off the shelf rated for 10,000psi and so perfectly rated to do so and safe at those pressure's.
Secondly, if for any reason it help's pursuade people to build hydraulic robot's and its perfectly safe then there's no reason to not allow it.

Hydraulic's arent like pneumatics if a pipe is broke. Unless you have an accumulator (which are banned) then a leak in the pipe will just squirt a tiny smount of fluid out untill the pressure is gone (being as the oil is non compressable then it is only a tiny amount comes out).