Yes, it does cause huge inflation, but that's not even the biggest problem with it. That would be: people do not really like to share fruits of their labor with strangers, so UBI would significantly undermine the motivation to do anything other than bare minimum.
UBI is not possible until robots and AI take over most jobs (but then we risk that one day the AI decides to just get rid of "those useless humans")
Isn't UBI just a sort of tax, which people pay already, whether they like it or not? I agree with your second paragraph though.
I don't think we should worry about the advent of AGI deciding to get rid of us; I'm more worried about the people who own the AI current AI infrastructure, as well as the current US regime, who don't see the value in the pesky humans beyond revenue and votes, respectively.