I run flail mowers on my tractors through very heavy grass for hours on end, and all of it is done on slopes. I mow capped landfills and drinking water reservoirs. I've had very good luck with my HSTs not overheating, I guess.
With an HST, there will be an additional cooler up front and if it is blocked with dust / dirt / chaff, or if any of the coolers behind it are blocked, than your hydraulic fluid will not cool effectively.
There's basically nothing to wear out in an HST, so short of running the hydraulic oil out of it, it won't wear out. Kubota's latest gen of Super UDT-2 synthetic fluid ships in all the machines now and works exceptionally well at both high and low temps.
Again, I think the Yanmar trans is pretty cool but I'm not sure it has real-life advantages over a quality HST.