UBI is far easier to understand. It's exactly what we have now (with perhaps higher taxes), except you get a cheque for $Xk as well.

NIT is a counter-intuitive trick to achieve the same thing within a tax bracket framework. Maybe it's more elegant, but understanding it requires a comprehension of mathematical concepts that many people (especially on lower incomes) simply don't have.

NIC would be harder to understand, or BI would?

If the latter, I wholeheartedly disagree. Normal taxes would remain, the only difference is that you get BI as well.

I meant that BI would be harder to understand... It would take more thinking to answer questions such as "what's the effective tax rate for net contributors?", "what's the level at which you're a net contributor", "if we switch to BI, will I become richer or poorer?".

The people who care about those questions are clearly already fairly well off, and they can afford to work it out for themselves. For the people who really benefit, only the last question matters, and the answer is obvious.