Not 100% sure what your asking....
As far as NV "breaking"- we go back and look at warranty work every year or so. In over 800 New PVS14s in the just over the last year, we have seen exactly One where a tube went out. Elbit replaced the tube under warranty in short order.
That's a pretty good ratio in my opinion.
I would honestly be more concerned about buying used as people can -and often do- do stupid shit with their NODs. The biggest thing we have seen the last few years is problems with the "sheetrock Rangers"- the guys that evidently never go OUTSIDE with their Nods but instead spend all the time staring at white walls inside their house looking for spots in the tube. It's ok. to do that, just don't blast your damn IR illuminator while your doing it!!! We have had several cases where a customer burned his own tube doing this. The IR is bouncing off the semi reflective wall and burning the tube. It makes a tell tale ring in the view usually.
For all the talk of NV being "a lot of money" (and they are!!), you see people do some really stupid shit with their units.
Most warranties are transferable, but if the last owner has tampered with the unit you may have issues.