All you need to do is to look at KSA or Qatar et al. and if those "guest workers" aren't a vast army of borderline serfs living under social fascist regimes then idk what would be a better example.
Those migrant workers form the backbone of labour upon which the cushy office jobs of the citizens rest in comfort and luxury.
One time, many moons ago, when I was still an anarchist (or at least in my AnCom transit arc) I attended a lecture with some utopian who was shilling UBI. I hadn't really grappled with a UBI seriously until this public lecture because it was very much a fringe idea at the time. At the end of the lecture, I posed a few pointed questions that cut through the core of the lecturer's arguments that I had come up with during the talk and (amusingly, in hindsight – maybe they picked up on something that I had not) the lecturer basically dismissed me as a tankie, except in a more polite manner but without actually addressing any of my concerns.
One of the key points that I made was that right now in the country I am in (and it's probably true for yours too) there's this grey market labour force that does the hard labour that most citizens refuse to do (at least for the going rate of those jobs as they are currently).
What we could expect to see under a UBI would be a hard nationalist turn, with citizenship becoming prohibitively difficult to exclude any prospective citizens from coming in and contributing to the overall costs of the UBI without necessarily contributing back into the system. Australia is a very good example of this, where migrant labour is borderline sharecropping with extra steps, and it's extremely difficult to get citizenship in large part due to whatever vestiges of the welfare state remains unstripped. The same can be said for the US particularly in the informal labour market coming from the southern border especially in agriculture, with companies like Tyson literally busing cheap labour over the Mexican border to exploit (in a liberal sense, not in a Marxist sense).
If we extrapolate this current arrangement out and acknowledge that either all of those shitty jobs at Walmart and McDonalds and so on would have to compete against a UBI to make it worthwhile for people to get out of bed and leave their loved ones and pursuits behind for 8+ hours of a high-stress grind (lol, that ain't happening) or otherwise those gaps in the labour market would need to be plugged immediately elsewhere. And you just know where that country is going to source its labour from and how it's going to do that because it's already doing it right now as we speak.
And then ✨ta-da!✨ you have a Qatar-like economic arrangement, except on steroids. Big fucken yikes from me on that one.
"It seems to me that I am more dialectical materialist than you, Mr Stalin."
— ~~H.G. Wells~~ Me, back when I was still an anarchist