It gives a family of curves, and as you say you have a knob you can turn...
I don't have a strong opinion about exactly what shape the curve should have, but I do have a strong opinion that it's probably important and I've noticed a tendency (first in myself!) for UBI proponents to be satisfied having checked the "it's a progressive curve" box without having looked carefully at whether it can produce the particular progressive curve we (more broadly) want.
Edited to add: I suppose if we do find that we need people to be taking home marginally more for the first N dollars earned, we could add a rebate based on reported income. Small fraud ("I reported a little more than I actually made") winds up pretty bounded in the amount it can walk off with; larger fraud ("I invented 500 people who are reporting income") isn't that much bigger a concern than it would already be with a UBI since the maximum rebate is probably much smaller than the stipend that would otherwise be expected. Note that I'm not calling for this, just exploring the space.
It's mostly just not that compatible with confiscatory-level taxes on very high income people, but the cause of the trouble there was never really the taxes (once they were paying the same rates as ordinary people). The problem there is regulatory capture and inadequate antitrust enforcement leading to abusive consolidated corporate empires.