I'm suspicious of the downvotes your comment has attracted - I suppose I'd like to challenge whoever is feeling so negative to explain why they think UBI gets pushback, if not the sustainability aspect?
I think everyone agrees that if we all got infinite amounts of free stuff that would be great. Failing that, finite amount of free stuff is good. There are good reasons that despite that we have a pay-as-you-use economy.
People keep trying to come up with financial perpetual motion machines where everyone gets lots of free stuff. Much like perpetual motion machines, there are broad principles the limit what is possible - people on average just don't do things if there isn't a reward for doing it, and we need people to first produce the stuff before it can be given away.
There are grave questions about whether a UBI can be simultaneously effective (ie, a meaningful amount of money) and also sustainable (ie, not catastrophically dis-incentivise working). It may be possible to do both at once, but that is really the only aspect of the debate worth talking about - otherwise it is a bit of a no brainier for everyone to get free stuff.