We have a hydraulic motor spinning a 19" auger on our rebuilt Big 12 cart.  It also has a floor auger which is hydraulic, and the two in parallel were a little too much for our outdated 32 gpm tractors to handle....they worked but weren't a whole lot faster then the other cart with a smaller auger.  We put a seperate hydraulic pump on the PTO (have two PTO track tractors now) and run one of the motors with each hydraulic source.  The tractor hydraulics can run the unload auger fine, or they have way more then you need for the floor auger, but the two together are a little too much for it if you are after top speed.  If we had a modern tractor with ~50 gpm availible I don't think it would be any problem to run from the tractor.  The same applies with our existing tractors if we had a single auger cart.

I think the complaints with the hydraulic unloading drives are related to buying an off-the-shelf one size fits all kit from somewhere rather then sizing a motor properly for the job and tractor being used.  With 32gpm at 2750 psi if I recall correctly there is a little better then 50 hp availible, so there isn't really any excuse for not being able to make it work, unless you are really swinging a heck of an auger.  The problem is the one-size kits have to have a motor small enough to get decent RPM with an old 20 gpm system so they undersize the motor and do not take full advantage of what a modern tractor has availible.