The only "advantage" to the donkee is the 3/4 HP 2 Speed. You can not get that from Innovative or Vertex at that price point. A 2 speed innovative spinner is going to be over $500 IIRC.
I've had the 3/4 HP 2 Speed model for about 14 years. I use it a lot, and the thing just won't die.
I've always felt the Donkee vibration thing has been overplayed. I have mine secured on a B&D workbench, and I've never had an issue with wobble while sanding. Balls I've refinished/refreshed dozens of times are still round. What a concept….
At low speed, the unit is fairly stable. There is no issue keeping consistent abrasive contact to the ball while it's spinning.
But what separates the Donkee (as stated above) is the high speed mode. It creates a speed that's simply superior for creating friction for cleaning, and burning in polish on tough surfaces. For these high speed purposes, I don't care that the ball sometimes wobbles a little. It's not going to hurt anything.
For me, the Donkee tradeoffs for the 2-speed make sense. I'll buy another Donkee when the one I have finally bites the dust.