fuzzy logic said:
Gulp! What's the 'obvious'?
Not a lot really...... maybe they're a bit big. Maybe a mite heavy - compared to the alternatives. The right angle drive is a tad "compromising" when attempting delicate tasks. In that a vertically-stacked layout with a modular EC motor mounted inline with a switchable planetary gearset to indirectly drive the platen in RO mode should provide better balance. Hopefully without any substantial increase in height. Sort of like the Metabo SXE 450 dual action rando (which unfortunately lacks a planetary mode).
The exhaust venturi, power module & switchgear could be incorporated into a horizontal handle. Yes, I know the big one also incorporates an extra auxiliary handle, too. I honestly don't think I've ever actually used mine. Ever. Some people may regard it as a necessity: to me, it's useless.
The low mounted horizontal exhaust airway & drive-belt housing (RO90 only) tends to cause accessibility problems sometimes, especially so in the 2 smaller iterations of the Rotex family. Plus the smallest RO90 seems pretty unruly & difficult to control at times, in direct contrast to either the much larger RO150 & much smoother running & more easily handled Deltex DX 93 E delta, Metabo's delightfully small SXE 400 or Mirka's DEROS 77 mm randos.
All the Rotexes are deliberately designed as "two-handed" sanders for safe operation, whereas MOST of the alternatives can also be safely operated single-handedy, with the exception of the big Makita, Robert Bosch & Kress/Mafell dual action machines, which are similarly "flawed".
None of which are in any way "fatal" flaws; but you did ask the Q. Which is (in part) why I don't actually have any Rotexes any more.