Normanb just posted a real life advantage. You cannot even begin to compare running a flail mower on a hydro to that of pulling a sub-soiler. I say, there is your real life advantage, choose to ignore it if you like.
You need to stop trying to compare the Kubota Hydro to the Yanmar's I-HMT. Saying it several posts in a row, don't make it so bro.
When you are backing a tractor with a 1200+ lb flail mower up a slope too steep to drive up, and you're doing it a hundred+ times in a row on a hot summer day with the A/C on in the cab, and the HST is performing well and getting the job done...yeah, you don't need to overly complex Yanmar transmission. Pushing the weight of the tractor and implement up that long steep slope a hundred plus times is easily the equivalent of working a ground-engaging attachment.