How does it take leverage from unions?
It would effectively be a permanent strike fund.
Wouldn't that help unions?
It's also not so much "taking" power, as it's not giving power you feel is your right.
Which, is the same kind of thinking that let's copyright holders claim every count of piracy is theft of money they never actually had.
If there aren't workers, there is no need for unions.
But that doesn't happen anyway.
UBI doesn't replace work. People still work. Pilot programs and tests show, people might work less overtime, or call out when sick more, so they can go to a doctor, spend more time home with a new baby, and stay in school longer gaining higher degrees. But they don't quit their jobs. So there will still be plenty of workers to join unions.