Have you not seen Pete Perron’s machines from a few years back? Here’s one thread about one of them. Works really well for a single-speed geared machine, much better design than the hunirex IMO. You can change the gear ratio fairly easily by swapping sprockets around.
As for shiftable-but-you-have-to-dismount-and-fiddle-with-stuff systems, that’s what Harper’s epicyclic hub (“blue shift”) was like, before the Schlumpf design added the foot shift idea. You had to get off, remove a bolt on the torque arm and reattach it to a different place.
What do I want from a geared hub? Most of my unicycling is xc, and sometimes it would be nice to be able to shift up and slow the cadence down on easy sections, but I just don’t think I’d use it enough to justify the cost. That’s not a dig at Schlumpf at all - making something for such a small market is bound to be expensive - but it’s enough to put me off buying one. If there was a multi-speed, light, reliable, less expensive option I’d be tempted, but I can’t see that happening in our small market. They need to be precisely made because of the need to eliminate gear slop as much as possible (not so necessary on a bike) and the need to be able to drive in both directions (most multi-speed bike hub gears rely on ratchets and one-way clutches to work, and could not be made to drive both ways just by locking the freewheel, and even the ones that do drive both ways like the Sturmey 3-speed fixed are very sloppy).