Hydraulic systems can be stupid dangerous. If you can't be positive it's depressurized, or are confident whatever work you're about to do will hold when it is pressurized, take the time to find someone knowledgeable.
Of a few other issues: even a small stream can more or less act as it's own hypodermic, putting really nasty (chemically and biologically) stuff into deep tissue. The resulting infections have killed people. I don't think it's like a death sentence but it can that sort of bug you think you'll sleep off and wake up dead.
I work around a lot of grosse stuff, but it doesn't get a head start from inside my muscle tissue.