Being that an HST operates as a hydraulic pumping circuit, instead of a mechanical connection (i.e. gears), if you get the tractor tipped too steeply, the fluid in the bottom of the tranny can't reach the intake for the HST internal plumbing, resulting in cavitation. Different makes/models of tractors would each have their own unique limits, of course, but it's something to be aware of with any HST equipment.
There was a thread here on TBN several years ago about some fella who took his brand new tractor way out in the West Va hills, got it stuck on a steep hillside, and couldn't get it out again, all due to this kind of cavitation problem. Never did hear if he got it out of there.
I sort of figure any hill steep enough that it might cause this type of problem is way beyond where I want to be going for more obvious reasons.