Quote taken from Bings website
http://www.bings.ca/index.php?page=s...ent=2&child=18
"We highly recommend NOT using aftermarket bolt-on spacers that have built-in wheel studs. We've personally experienced at least 3 stud failures with H&R, Eibach and other brands. ONLY use ARP wheel studs and direct fit spacers like these ones. Everything else is dangerous."
I know plenty of people that use bolt on spacers such as PBM without an issue.
With that said I believe either one can be safe
if done correctly. Which style you go with depends on how far you want to extend the wheel. Hubcentric slip-on style is good for up to 15mm. Bolt-on is good for anything above that.
I think so many people have issues with slip on spacers because they do not press the extended studs in all the way. I've witnessed this on two different cars that were snapping studs. Looked at the back of the hubs and found some of the studs were not pressed flat against the hub.
Either that or people are using cheap wheel studs or the wrong bore size spacer.