the only impact of a properly designed spacer is the increased load on the hub from leverage. And as said above that is exaclty the same as moving to a lower offset wheel.
much more important is "hubcentric". basically a wheel stud's job is to clamp the wheel to the hub, not locate it correctly. You must have a "hubcentric" fit between the wheel and the hub, this means that the wheel should be held in the right place by the hub not by the wheel studs. If you can put the wheel on without wheel nuts and rotate it a little it is not hubcentric.
basically a hub has a ring in the middle, and the wheel should be a tight fit on that. if you add a spacer it must fit snugly on the hub, and the wheel must be located on that as well. I have never seen a slip on spacer that has a hub ring to locate the wheel.