"Used" has a thousand definitions. Some units get thrown down flights of stairs, and tossed in and out of the back of crappy vans that smell like socks and farts. Others never see the outside of a home studio, and get carefully enclosed in some sort of case after each use. Some have a light foot, and tap the switches gingerly... others are like an angry Sasquatch stomping on them like they're trying to embed the unit in the floor. Thus, the only useful predictor of hardware failures is exactly how "used" the unit you purchase actually is, and that is often very difficult to ascertain unless you're buying it from someone you know and trust to honest about it's history.
Fwiw, my Helix floor is 7 years old. I've only gigged sporadically over the last 3 or 4 years, but for the first couple or 3 years of it's life it was on stage quite a bit. The rest of the time it's been in my home studio, but used pretty much daily. I don't abuse my gear, but I freely admit that I don't baby it either. The screen is in good shape, but there are some light scratches on the housing here and there. All the footswitches, buttons, and knobs work just fine, and always have. Like everything else in life, ymmv..