Spacers position the wheel where it was not intended to be by the vehicle designer. That means additional loads on wheel bearings, additional loads on suspension components, and in the case of the front wheels, different steering geometry. Spacers also introduce an additional failure mode because the mounting point can have a shear plane that doesn't exist in the standard wheel mounting. If they are well machined, the risk isn't large, but it is still there. If they are not well machined, given that you are increasing the load on the wheel studs by a large factor, you can have problems with studs breaking and in the worst case, wheels coming off. That applies particularly to any spacer that you bolt through from the wheel to the original hub. The load on the studs changes as the wheel rotates, so you can get fatigue failures of the wheel bolts.

The safest method is a spacer that is bolted on to the hub to eliminate the shear plane. That doesn't solve all the problems, but it eliminates one failure mode.

I will not use a spacer on my own vehicle due to the risks involved. If I had to use the wrong wheel, I would have it machined to fit the hub bore.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue