By this response I'm guessing your view on HST is based on lawn tractors. With most of these new hyrdo tractors having a multi-range sub transmission, I don't know of many that can't adequately pull any hill. My B3200 will pull my bush hog at full speed in M range with no problem, even on steep hills (we live in the mountains also).
I'm in the Rockies. I have rented 25HP deere HST's from the rental yard when i need 4x4 and my old 9N will outpower them. The Hydro's suck at least 25% power from seat of the pants feel. I am an EX farmer and i like simplicity. Hydro's are not simple in my book. FWIW i will be picking up a flail mower as a bush hog is inherently dangerous in my line of work and leaves a crappy looking job with windrows.......