I personally know one person who had a spacer failure. It's my buddies brother-in-law. Switched from his winter to summer rims. Front pass. spacers lugs where only fingertight. Threw the wheel on and buttoned it up. I don't know how far he went before the wheel fell off, but it wasn't far.

I'm pretty sure that about 95% of the issues with spacers are caused by knuckleheads like that.