Dont forget that with a UBI system, it is not simply that everybody is $12k better off. It needs to be arranged against tax reform so that there is an income level which stays the same. Below that level, you would be better off. Above that level worse.

I'm not using actual proposed values, just an example. Lets choose $100k as the pivot point.

So person earning that currently pays $30k tax, meaning they get take-home of $70k. The new system means they get $12k UBI and tax needs to leave them $48k so they stay at the same salary level. They now pay $52k tax.

That means 52% tax rate, across the board.

The guy who previously earned $20k and paid $2k tax had a take home of $18k. He gets $12k UBI. Now he pays 52% tax on the $20k, so $10.4k. His take home is now $21.6k.

The closer you get to the floor, the better security there is. You can live on the $12k but you are not rich, and nobody goes without.

The $1m high earner should have paid $400k in tax but actually paid $100k because he pays $20k to an accountant. He now gets $12k UBI and pays $520k in tax because income tax rate is flat and the accountant needs to find another way to earn a crust, though he gets UBI, so..