I haven't used the Q128 (I'd like to), but from what I've read, it seems like a Q100 on steroids and I like the idea of a more power-capable motor that retains the sm. diameter that hides behind the cassette/.free wheel. But that sm. diameter means extra width to achieve stator area and like other minis, that can create some fitment issues.
But not what you are thinking of.
To digress a bit, as power goes up, there becomes less need to shift gears and high number gears sets are not needed. Even w/ my low-powered Q100C, I seldom use more than 3 gears (the 4th on the steepest hills). But having said that, I know installing a 9-speed cassette on a Q100C brings the motor width out to 137 M/M, which means the chain stays need to be spread slightly and an additional washer installed between the drop-out and the cassette spline end. This is NBD.
The issues I've had have been because of housing width, not drop-out width. W/ my MXUS mini (in between the Q100 and the Q128 in size), the sloping side cover would just touch the brk. caliper and spoke heads and I had to grind them down a bit.
The Q100 w/ the free wheel is not too bad, but w/ the Q100C w/ it's long cassette spline, the center-line of the motor is off to the side and the whl. has to be dished alot to bring the rim back to the center. On the three bikes I've installed Q100C's, one I just couldn't get the wheel dished far enough to make everything line-up. Not every bike frame is built on the same jig, but probably it's fairly rare for the Q100C not to be able to fit.
My Ezee's (same size as the other 5 Kg. geared motors, BPM, Mac, etc.) are a snap to install, they just fall in w/ no dishing.
