The "classical" UBI argument from a liberal point of view (classical liberal, not US liberal) has typically been that UBI would lower the complexity and by extension cost of welfare by removing the needs to means-test. In Europe, UBI was typically more likely to be pushed by (by our standards) centre-right parties.
For this reason, UBI traditionally was seen negatively by the left, who saw it as a means of removing necessary extra support and reduce redistribution.
Heck, Marx even ridiculed the lack of fairness of equal distribution far before UBI was a relevant concept, in Critique of the Gotha Program, when what became the German SPD argued for equal distribution (not in the form of UBI), seemingly without thinking through the consequences of their wording, and specifically argued that "To avoid all these defects, right, instead of being equal, would have to be unequal".
Parts of the mainstream left today has started embracing it, seemingly having forgotten why they used to oppose it.
FWIW I'm far left and in favor of UBI, but one thing to keep in mind is that while it's a simple concept, details (i.e. amounts paid and amounts taxed) matter a lot. There's no reason why UBI can't be redistributive if one desires it to be so - you just use more progressive taxes to fund it.
And, of course, reducing the complexity and cost of welfare ought to be a left wing talking point as well! Again, it depends on what you do with the savings - sure, it can just be taken and used elsewhere, or you could maintain the spending but raise the bar on how much UBI provides.
For this reason, UBI traditionally was seen negatively by the left, who saw it as a means of removing necessary extra support and reduce redistribution.
Heck, Marx even ridiculed the lack of fairness of equal distribution far before UBI was a relevant concept, in Critique of the Gotha Program, when what became the German SPD argued for equal distribution (not in the form of UBI), seemingly without thinking through the consequences of their wording, and specifically argued that "To avoid all these defects, right, instead of being equal, would have to be unequal".
Parts of the mainstream left today has started embracing it, seemingly having forgotten why they used to oppose it.