The answer to the question is Yes & No.
Yes-
A
properly designed, built, and installed set of
reputable spacers (such as SpiderTraxx for example) should act as an extension of the wheel hub and be a solid mounting point for the wheel. It's unlikely to shear the studs off anymore than a stock wheel would on a stock hub, nor will it just fall apart on you.
No-
An improperly designed, built, and/or installed set of reputable/questionable spacers (such as "eBay specials," no-name brands, and SpiderTraxx) can be extremely unsafe and very dangerous. They can be machined out of round, the holes not drilled true to circle, the reliefs not deep enough, cheap studs and/or hardware used, inferior aluminum, not torqued down enough, not retorqued, ect. This can cause a catastrophic failure of the components related to the spacer as has been noted in the past (one member installed theirs improperly and lost a wheel.)
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Long & Short of it is, that for the most part if you're running a known good unit, have had it installed correctly, and are checking torque as recommended; they should be plenty safe (we're not going into pros/cons/reliability concerns/ect. strictly talking from a safe or no safe point here.) But I have seen many questionable brands, improper installation, and lack of follow-up after installation... and quite a few of those haven't resulted in tires flying off (surprisingly.)
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borfoo3
What brand were those? They don't lack a hub centering ring like the SpiderTraxx have. I could be wrong, but a lot of the pictures I have seen of SpiderTraxx installed shows that they seem to have a deeper relief so you don't need longer studs. (Also I wouldn't say those fronts were going to fall off, you'd be surprised with how much mechanical strength just half the threads on a good nut have; though I would not be happy with that on my own vehicle to be honest.)