Spacers, like wheels with wider offsets, also affect your steering and suspension geometry. In particular, it makes your vehicle more susceptible to bump steer.
In an "ideal" set-up, the steering kingpin is in line with the centre of the tyre - i.e. your wheel wraps far over the hub, and when you turn the steering wheel, the wheel pivots around the centre-line of the tyre.
If you offset the wheel (imagine an absurd spacer, say 50cm thick), when you turn the steering wheel, the road wheel describes a big arc around the kingpin, which is 50cm from the actual tyre. As it turns left or right, the wheel moves forwards or backwards within the wheel-arch.
By the same token, if that wheel hits an obstacle while you are travelling straight, there is 50cm of leverage pushing that wheel backwards within the wheel-arch, putting a LOT of strain on the whole steering train.
Having said that, if there is a good reason for wanting more offset, the spacers are probably OK, if they are well engineered. You might want to check if they are legal or if there are any insurance implications...
Regards,
Michael...
www.expeditionoverland.com