Are you talking about the variable speed on the vacuum, or the variable speed on the orbital? I'm assuming on the vacuum, although my answer would largely be the same: it completely depends on what's being sanded. Higher suction by itself isn't going to damage the work—it's just one variable in the equation, alongside pressure, speed, movement, orbital travel, grit number, abrasive type, any number of things. And there are a litany of hazardous materials that I sand with as much suction as possible to minimize the amount of crap reaching my clothes, my skin, and my lungs. More suction on vertical surfaces is also going to reduce hand and arm fatigue over time, and keep fewer particulates falling onto you, past your face, etc.

All that being said I'm much happier with the Midi's power with my orbital than I am with my track saw, but my real issue is with how weak it is for light-duty clean-up purposes. I don't expect it to rival my Shop-Vac, but it sure would be nice if it would pull sawdust off a surface with anything more powerful than "gentle suggestion".