I had a prototype and had no problems with the controls. I tested and used that one extensively and documented it on my YouTube channel. The other winners of the FrankenZeros didn't report any issues either.
I also ordered two SE models and one Kickstarter unit, so four in total. I don't use any of them on a regular basis but I took a KS unit to PAX South and the few friends who tried it didn't have any problems with the controls. For the SEs, one is in the original box and I haven't done anything but turned it on to make sure it worked. The other was gifted to a friend on the other side of the country, and he did report eventually having a squeaky, sticky D-Pad and buttons.
My point being, the evidence is on my channel, I don't think the prototypes ever had problems. I've never experienced the sticky button/d-pad problem first hand, but there's been enough reported issues to know that a problem does exist. I have to ask though, has anyone made a video showing the problem? If there is, I've never seen it. I find it hard to believe that no one has documented it on a device that includes an on-screen hardware test right out of the box.