I have only ever had one Smartboard really break down, and that was very old and had taken a fair pounding over the years ( I work in a special needs school). .It had an area that did not respond to touch. It was part way through the replacement project, so I just swapped it with one we had taken down in another room. I did not even consider trying to fix it. We have replaced all of them now with new technology (touch screens), but that was for other reasons than reliability of the boards.
When we replaced them, some even still had the old serial ports rather than USB.
I have has many issues with disconnected boards, and of course, projector lamp failures, and some interesting discussions with teachers over the effect of using blue-tac to attach papers to the board, or leaning displays on them, and then wondering why the computer mouse did not work, but all bar one were working when we recycled them,