I think you guys are confusing spacers with adaptors, just spacers will never fail, made from aluminum or steel. Adaptors made from cast aluminum or pot metal could fail, but billet aluminum is unlikely to fail.
But for adaptors I think I would prefer steel even though a stud pulling through a billet aluminum adaptor would be very unlikely if its tightened properly.
And huge wheel offset would be a lot tougher on spindles than just spacers.
