Jipe

I am in Belgium and I know many people having a Schlumpf, its very common on recumbent and velomobile. I have myself a SD on one bike.

Losing one button is very common even when correctly fastened. Its good practice to very it regularly.

For the rest of the drive, its indeed very reliable and need very little maintenance.

But both MD and HSD have the drawback of a noticeable lower efficiency on the 2.5 times overdrived for HSD, 2.5 time reduced for the MD position. They have also the drawback that a difference of 2.5 time between the two positions, its very big, it force to change simultaneously several gears to maintain a similar pedaling cadence which isn't easy with a cassette+derailleur (its easier with an IGH like the Rohloff).

The SD is better because the difference between the two positions of 1.65 is relatively close to the difference between the two chainrings of a compact 50-34 or 52-36 and the loss of efficiency on the overdrive position is smaller.