While digging out a truck, I blew a bucket cylinder hose. I was working the little tractor hard, and the bucket curl let go. I don't remember it being a drop, as it was close to the ground, it just wouldn't respond. The hose actually completely seperated, so it pumped a good amount of fluid out. I saw the fluid stream and shut down the tractor.
So I suppose its perfectly possible that a hose could blow and either the bucket dumps you out or the boom lowers you. But from what I have seen it moves slow enough for the person to be able to react appropriately.
I won't fear a hydraulic failure but more importantly a tire failure. I was using forks and a palette to load 45' trailers with our equipment last fall when I must have hit a nail with the front right tire. I had raised the load and set the edge of the palette on the edge of the open trailer, climbed in and began unloading the palette. When I was done, I hopped back in the tractor, started it up and thankfully lifted before backing up. As I lifted the right tire gave way and the palette slid into the trailer wall. Had I backed up, I think it would have tipped over.