HST transmissions are wonderfully convenient (I have three) but not efficient.I don't plow with my tractor, but I use a box blade and manicure the driveway with a plane. I am more of a sparky engineer, but I wasn't completely asleep in class when fluid dynamics was discussed. I think (don't know from experience like I'm sure a lot of people on here do) that the major drawback of an HST over a gear would be the heat generated in the hydraulics. Too much heat can lead to things like blown seals etc... HST's have infinite speed control in 2 or 3 ranges, but this can be a problem. More flow should help mitigate the heat buildup and in order to get more flow you have to push the pedal all the way to the floor (assuming the RPM is constant). If plowing could be done in low range with the pedal to the floor, I think this would be the best case for heat mitigation (is this enough? I don't know). Does anyone have experience doing this? Even at this, I could see where a gear would give more options for speed control.
This, of course, is why high HP tillage tractors do not use HST transmissions, but rather other, more efficient and more expensive variable speed alternatives.
FWIW, MAN tried marketing HST railroad locomotives in the US in the 60s. Yes, they would pull trains but they were a commercial failure do to inefficiency (fuel usage) vis a vis the electric transmissions universally accepted the world over.
IHC marketed HST tractors in the 1970s in HP ranges exceeding 100 HP. Again, these were commercial failures for multiple reasons, including fuel usage. Yes, there were other reasons.
Only recently have manufactures again marketed HST tractors in the 50 HP range, and these are certainly not directed at the tillage market. There are valid reasons for this.
SDT