I realize this is a fairly old thread, but with some recent discussion still happening I wanted to chime in just a bit..
If done properly, wheel spacers are an absolutely safe option, both on and off road. I won't claim that ALL the failures happen due to install error, but it certainly seems that a large number of threads I have seen come up across various boards certainly point that way. Between torqueing the nuts way over spec and stretching the factory studs, to installing the conical nuts backwards, to not cleaning the mating surfaces properly and lastly, not removing the existing loctite when removing and reinstalling the spacers.
The first batch is pretty common sense, however, I recently ran across a discussion of a possible failure point being existing thread locker that was not sufficiently removed in between taking off the spacers and re-installing them.
There was a guy that lost a wheel at highway speeds after running spacers with no issues for a long long time, and when his studs sheared off on him at 70+ mph he was able to determine that there was still some old thread locker residue that could be seen on the failed studs. Now this isn't 100% confirmed, but the only conclusion that sounded reasonable is that if you remove existing spacers that were thread locked and do not chase the threads to fully remove the residue, there is a chance that you are either over or under torqueing the nuts going back on, while also possibly leaving just a tiny amount of space between the threads allowing vibration to eventually cause the studs to fail.
Long story short.. I just did the same thing and failed to clean my threads. I will be in the process of removing the spacers again and literally chasing the threads with a die just to be on the safe side. I am 100% pro-spacers, but am petrified of the idea of losing a wheel at highway speeds.. especially having a RTT permanently on, running a high lift with 35's and no front sway bars. That certainly would be a recipe for a rollover.