I bought a rebounder trampoline 7 years ago when diagnosed with osteopenia in my spine. I thought it would be gentle on my joints and that I'd be a better jumper. It is gentle on the joints if you watch your form, but it makes jumping on " land" feel very leaden. To get bone benefits I think it has to be used pretty vigorously, and for a long time. This can, even with music and multiple "moves" get tedious. I usually managed about 15 -20 minutes, not probably enough. I still use it occasionally, but it now lives on its side in the garage because the floorboards in the house were starting to squeek...
I now have osteoporosis in my spine and osteopenia elsewhere. I think I recieved more cardiovascular than bone benefit, but who knows.