If your spacers fit snuggly on your hub and have a new hub built into the spacer for your rims/wheels to mount to then they can be safe. If your spacers just ride on your studs and cover your hub so the rim is not mounted to it, then I would say that you have made a poor choice in spacers.
The hub is the ring of metal that passes through the brake disc and the center hole of the rims snugs over it. It does two things: It supports much of the load of the vehicle and it causes the wheel to be centered on the car.
Many non centric spacers cover the hub so all of the weight is carried by the lugs. This stresses the lug and can allows the rim to be mounted off center. If the rim is tightened up off center the indication can mimic the wheel being out of balance or in the worst case can give you the wash board effect of riding on a corduroy road.
Pot holes can cause studs and wheels to break when everything is perfect, a poor quality spacer can increase stud and rim fatigue and failure. Tire dealers and alignment shops have seen these issues, so they are not going to cover any material used with a spacer.
If you have non hub centric spacers, don’t be fooled by the fact that you have not had a failure. It takes time for the small amount of movement or mis-alignment to fatigue the wheel studs or rim but the damage is being done.
Cheap universal rims that are not hub centric and just hang on the lugs can cause the same problem with or without spacers.
Hope this helps.