I have "Z" or "schottle" drives on my Tito Neri.
The mk 1 units had a problem with the gears stripping when over driven by 900 series Graupner motors.
The gears are now made of sintered bronze and accept the 900 series power without problem.
Their use is a matter of expense. The units cost an arm and a leg, 900 series motors are not cheap either. The servo has also got to be a top class high performance jobby as well, to move the unit against the thrust of the prop sometimes.
Users in the USA, reported failures in some of the unit structure, but I have had no problem there; I think it was one of those self perpetuating faults, technical experts solving a problem that didn't exist.
They really do do what it says on the tin, nothing else like them; that is if you need them.
BTW a Futaba/Robbe F14 Navy twin stick is a must to get the full benefit, especially if you fit an extra twin stick on the right hand side!!
See what I mean about the money!!