I think one problem with NIT is it would likely pay out annually, when income taxes are filed. That's not very helpful if you're living paycheck to paycheck. UBI can more easily dish out money on a more frequent basis. Additionally, this was already mentioned elsewhere, but, a major argument for UBI is that not having any sort of means-testing will drastically streamline the process, reducing costs and increasing efficiency. NIT still involves means-testing.

Looked into NIT a little, and as I understand it, it could basically be a linear function where Y=0 when X (income) is, for the sake of example, $40,000 yearly. Then anyone over that pays increasing income tax while anyone below receives increasing amount of money from the government instead?

Apart from whether linear actually makes sense or not in the real world.

It doesn't have to be linear, that's just the easiest model to visualize and understand it just should be smooth enough in a way that ensures no discontinuity.