It's difficult to bring up the hydro/gear transmission subject here without starting some very heated conversations as the flames grow high! /forums/images/graemlins/grin.gif I'm not putting down either type of tranny - I feel they both have their place so please take this info in that spirit.
This has been discussed before, but to try to summarize in a nutshell why the 'wisdom' is that a hydro may not be the best choice for ground engaging duty...
Any transmission absorbs power. Fact of life. A fluid connection is not as efficient as a mechanical one can be. Fact. That means, that for two otherwise identical tractors, one gear and one hydro, the hydro will put less power to the wheels than the gear. Not talking about the minor difference you'll see listed for PTO power. That's only the minor loss caused by the hydro's pump. I'm talking about actual power transmission through the transmission to the axles. (drawbar horsepower). Now since most CUTs are probably 'overpowered' for their size anyway - and most owners are reluctant to ballast the tractor for absolutely maximum traction - this small loss of pure efficiency probably does not actually impact 99% of CUT owners. They've got more power than they can put down on the ground anyway (traction limited) and the small amount of extra fuel use is not a big deal either.
The 2nd issue with the hydro comes from the first. That loss of power in the fluid transmission doesn't just dissappear into a black hole. It's being converted to heat. Several thousand watts worth. When the tranny heats up, it isn't doing any favors to the internal seals and o-rings. If it gets hot enough, it will actually begin to change the characteristics of the transmission fluid - among other things increasing fluid shear (the source of the power loss in the first place) - and that leads to yet more heat. So - on paper, push a hydro hard enough to overwelm it's cooling system's heat-shedding ability - and you could cause problems. Now, like the first situation, is it ever going to be bad enough in the real world to actually affect a typical CUT owner? No one seems to really have hard statistics at hand and a lot of people seem to have pushed their CUTs pretty hard without apparent problems.
So - the ground engaging thing is real enough - in a lab or on a test stand (rolling dyno) - but it may very well be debatable as a source of real-world problems for most CUT owners in the field.