UBI becomes the single biggest election issue. Politicians promising increases to get elected, and all the bad policy that stems from such motivations. This is then followed by an inevitable pullback at some point in the future, causing immense harm to everyone relying on every cent of their UBI (most people).
I'm still unsure about UBI, but isn't this already a problem with taxes today? Politicians promise lower taxes.
UBI is one of those ideas that is theoretically awesome but doesn't survive contact with reality, or rather the inevitable flawed humans that administer it.
You don't know that - you just base it on your theory.
Of course, every idea gets perverted by current political system, but the problem is often not the idea itself. Huge appeal of UBI is that it's simple hence easier to control. Taxes were supposed to be simple too and yet they got overcomplected to serve the special interests. The danger of abusing UBI for political gains is as real as it happened with taxes, but it doesn't mean the idea is inherently bad - we just have an inefficient and harmful government system and fixing it should be a priority if we want anything nice like properly implemented UBI. Another Yang's idea, Ranked Choice Voting could at least mitigate some of the issues like political bipolarity.
I have yet to see a government institution remain uncorrupted and focused on the original principles upon which it was founded. That has little to do with politics as every party comes up with some bad ideas once they find themselves with power. This is at least partially due to the fact that every party is under constant pressure to 'do something' even if doing absolutely nothing (or at least nothing visible) is the right choice.
I have a very hard time conceiving of long-term solutions to such large problems that don't basically boil down to 'if the government is in charge of it, it will end poorly'