Well, so they're still nothing more than a way to negotiate collectively, even if the employer is a weak negotiator, or if the union reps make poor judgement.
As long as the employer consents, and the union reps consent, and the majority of the union approves a bargain, then I don't think there's anything inherently wrong with getting paid to not work.
Depends on the union and how long it's been in operation and their goals. Some unions demand and get ridiculous perks. You may have heard of the "rubber room" - a.k.a. paying employees not to work. That happened.
https://www.wsj.com/articles/SB114118143005186163