They're safe I suppose, but lemme tell you my story. I had a set of 16s that bolted directly to the hub and fit. They have Dunlops on them, and the fronts are full of tread, but when I took them off the backs were bald as ****. I could whip that car through corners at speed with 0 fear of sliding or anything. Well one of the tires popped cuz the belts came through. So I ordered a set of Honda offset 17s with wider tires and adapters. With brand new tires, I am afraid to whip it around corners, it doesn't handle nearly as well. Maybe it's just the tires or somethign I dunno, but I do NOT like the way it handles now. Runs great in regular driving though.
I have wheel adapters on my 77 280Z. 16x7' Rota slipstreams, 225/50/16 tires, and I'm very happy with the performance, and don't feel at all uncormfatable driving the car hard. This car with the adapters will see track time. Every so often I do hit the adapters with a torque wrench just to be sure they're not coming loose, so far they've not loosened at all, and I don't have any fear of them breaking.
If you have reservations about using wheel adapters/spacers, just look at the semi trailer trucks running up and down the interstates. Most of them have some type of spacers between the dual wheels to connect them. If spacers are safe with the speed and loads those brutes are running, you should have no worries.