29 degrees isn't any slouch of a hill -- I sure know that when I have been on equivalent roof pitches (7/12) or ski slopes, it's enough to make me adjust my foothold and stance quite a bit, and not fool around. I would want to test any UTV/RTV before committing to the purchase.
If you shop for a Kubota RTV, this would suggest skipping right to the higher HP models (RTV-X 1120 or 1100). The Kubota HST will be a help in terms of engine braking coming back downhill, as you may not want to be riding wheel brakes heavily.
The Honda Pioneer has a real transmission (geared or auto) and can transmit engine braking through the wheels too. Not sure about the other brands, but generally the ones with a belt and/or CVT cannot apply engine braking, so you'd be relying on wheel brakes alone. Not something I'd want on long downhills.
If you are used to HST on a tractor, the Kubota will feel natural and confident on hills. But I still would want to test it on a similar slope to say for sure. I have done steep slopes in my RTV-X1120D, but only short ones, as part of a rolling trail. It handles that fine, but that is not the same as a long continuous slope.