Magura Hydrostop HS 33, is THE hydraulic rim brake, have been made for many years, in Germany.

they work fine. V brake boss mount, closed system . [red compound pads come from KoolStop,Oregon]

of course , as I understand Magura's Gustav is their Hi G Disc, fit a pair of 210 large discs,
and you can fling yourself over the bars on any slope.