I kind of love cycloidal Gearboxes, they're a similar concept to a harmonic drive but without having to worry about the flexible spline. First read about them here
http://lcamtuf.coredump.cx/cycloid/ and now really want to make one.
From what I can tell the main disadvantage for FRC COTS is that they have to be designed for a specific ratio. Unlike a planetary where you can combine a couple lower reduction stages for a variety of high reductions the manufacturer would only be able to offer a few high reduction ratios.
That aside, I really want to make one for a bag motor to steer a swerve, would get rid of the really tall vex planetary problem 696 encountered this year.
To add to pros: they can also transmit lots of torque because 1/3 of the rotor is engaged at all times, and the vibrations can be dealt with for the most part by a second rotor 180 degrees out of phase.