I'm running a 160mm cable BB7 on the front of my roadrat (until recently without a rear brake). I've not had any fade problems using it as an (occasional) drag brake, although the hills round here are more the short and sharp type. You must have some very long hills round your gaff (or perhaps not be comfortable at high cadence for very long?).
Fluid boiling is the main risk with a hydraulic brake - you suddenly have no brake if that happens. A larger, heavier rotor helps a bit, as does changing the fluid regularly. We do have some hills where a rim brake will start to burn if used to hold 30mph, and while that isn't serious in itself, the front tyre pressure could be getting very uncomfortable. Coming down off Uffington White Horse is a pretty good test of any brake, since it's way too steep to pedal and you also have to come to a dead stop at the end.
The tandem crowd like the Avid cable brakes because there is no risk of fluid boiling and it keeps the heat away from the rims. Apparently the plastic fittings melt on long descents, but Avid sell spare parts