What I mean is it's another component to fcuk up or fail.
I'm no expert but slip on spaces can't be as 'safe' as having no spacers. You put more shearing loads on your bolts, and because things are never prefectly machined you have scope for 'play' etc and eccentricity, precesion, may lead to vibration and metal fatigue. As for bolt ons now you have more bolts to fcuk up, come loose etc. I just don't trust 'em basically. Don't want a wheel flying off at 100 km around a corner. May be ok for show cars though.