I'm not sure that's true, though most policies are implemented that way. If you get, say, a basic income payout that's a function of the income you make, that function scales all the way from 0 income/max UBI to >= max UBI income/0 UBI.

Not that that's like ... a policy slam-dunk or anything. Even putting aside the massive hurdle of getting UBI enacted into law, you'd still have to cope with sources of unreported income, asset/gift valuation, and so on. But that second category are, at least, tractable problems as evidenced by most countries' tax codes having contingencies to deal with them.