Wheel spacers are perfectly safe. This assumes they are solidly made - which I would assume they were.

I look at a wheel spacer as simply making the center of the wheel thicker - which is a good thing.

Don't use the cheap pot metal "Cal-Custom" spacers. They are junk and will break. When the break the wheel can't be tightened evenly. What's worse is you probably won't know they broke.

On my Duster, I had 1/2" thick spacers made (water jet cut) and attached them inside the wheel with screws so there would never be a risk of misalignment. They are still working after many hundreds of runs.

Paul