it's actually easy enough to understand why anti-poverty advocates would oppose a ubi: they'd be out of a job, and possibly on it themselves.
i posted a discussion with david graeber a while back where he points the situation out pretty clearly. a ubi is going to eliminate thousands and thousands of government jobs. but, anarchists are about getting rid of governments, not creating pointless jobs.
that is the difference between myself and paul krugman. i mean, i'll take keynes if i don't have another option, sure. but, i'd rather tear the system down than prop it up.
and, this is really just a reflection of how depraved the status quo is.