EEEEEWWWWWWWW!!!!!!!
Rubber spacers and twist-in spacers?
Run away! Run away! Run away FAST and screaming! (IMHO).
I have seen springs after running those things for a few years. Whats the old adage? "Pay now or pay more later"? Yeah. Good way to toast your springs.
As for safety, they have to be ok or they would not be marketable (we would like to hope), but they are still not good for your springd or your ride quality. You would be much better off just getting the springs. For the cost, why mess with it? Yes, spacers are dirt cheap, but springs are not that much, and you have to do nearly the same amount of work to put either in.
I cataloge cheap spacers (not to be confused with high-quality alloy spacers engineered to work safely like you find in certain lift kits) in the same vein as torching coil springs. Not cool and potentially dangerous. (Ya wanna spaer your springs with bits of cast iron and rubber, then go catch a killer jump in the dunes, just to have them fall out? Seen it!)
Just my 5 cents worth. (as someone else said...inflation.)