I don't know if I satisfy your criteria, canali, but having tried out the trainers, I can attest to their usefulness as a balance-honing device for skiers. I have had two (dryland) sessions, moving from green to red, and I look forward to sharing my insights about the process and tangible benefits, but of course the real test will be on skis on the snow. I want to play with them a bit more before writing a review, but I can say right now that it will be positive with some caveats.
*I received a Sweetspot Ski Trainer set from the company in order to review them but I don't know of any other personal reward than (I hope!) getting better at skiing.