They've been legal for a long time, and they're not hard to build or use. They are, however, slow as death.
We sped ours up from this initial test, and it was still far too slow, and we'd have been better off going with pneumatics.... The fine adjustment ability was really cool (and led to the first successful autonomous scoring our team had ever done--and it was killer consistent), but massive overkill for the game.
https://www.youtube.com/watch?v=gTLMkUmhU2Y