I recently picked up an old Cannondale MTB with a pair of Magura hydraulic rim brakes. I have never, ever had rim brakes on any bike that work as well. Very powerful, easily modulated and smooth as glass.
Has anyone tried them on a tandem? I know the arguments against: more complicated; not field-serviceable; relatively fragile piping, e.g. I'd love to hear from anybody who has actually used them. The poster of a very recent thread was concerned about how often the cable brakes needed to be adjusted. Hydraulic brakes generally adjust themselves. Rim brakes also don't build up as much heat as discs, so fluid boiling should not be an issue.