It would be no different than e.g. the existing payroll tax, or income tax withholding. You'd just notice it as a reduction in your paycheck. And if your income is high enough, the UBI can simply be a tax credit.
We can use the system that we already have, that is, income tax. It has worked fine for quite a while.
Not saying these issues can't be addressed, just pushing back against the notion that UBI is simple to implement.
I think the desired effect is simply a refundable tax credit, and then you can adjust the existing progressive tax rates as needed.
In the model proposed by Milton Friedman, you start with an exemption amount, and a specific proportion of any unused exemption is refundable.
The first dollars you earn reduce your exemption, but at least they aren't taxed, until earnings equals the exemption amount, at which point you are getting no subsidy and your next earned dollar starts being taxed at the lowest rate.
The effective tax rate on those early dollars would be equal to the subsidy rate. For example, an exemption amount of $20k and a subsidy rate of 40% would mean earning nothing would grant you a $8k credit ($20k * 40%). Earn $10k and you would get a $4k credit ((20 - 10) * 40%). Earn $20k and you would get no credit and pay no taxes. But effectively your tax rate on the first $20k earned is 40% -- because you earned $20k and only netted $12k in additional income.
If instead you just give everyone $X, and just have the first dollar earned get taxed at whatever the lowest rate is, you don't have the same problem with regressive rates.
It is not the case that you can reasonably just give large groups of people with histories of poor financial literacy cash as a replacement for food, shelter, or other necessities. Many people will make bad choices if just given money. That's why many of these programs were built this way: to ensure people can't spend all their cash and be unable to afford baby formula or rent.
I don't think anyone doubts that often people will spend additional money they are given productively.
I'm not trying to be unreasonably judgemental, and I don't mean things like bond rates or yield curves. Things like making sure you have enough dollars at the end of the month to make a rent payment. Making sure the check gets written so it actually happens. Keeping track of automated withdrawals and ensuring the number stays above zero with the timing of all of them.
This is not trivial at all for a huge chunk of the population that is served by social welfare programs. You can't just expect it to work out giving them cash instead.
The vast majority of poor people do just fine with money when given to them. The 1-2% who are addicted to substances or mentally ill will always need additional services, but why throw out the huge efficiency gains in just direct cash transfers to all because of the worry that 1-2% might waste it? The efficiency gains of a UBI vs. most means-tested programs is likely more on the order of 20-50%.
A similar argument is “would we give a UBI to billionaires too?! What a waste!” Yes, what a waste of 0.0000003% of the UBI!
One shortcoming of the UBI studies I've heard about is that (naturally) they focus on settings that I don't have much confidence would necessarily translate to other settings like the current US. I can easily imagine the efficiency of UBI changing over time, let alone across locations, due to a plethora of factors. This isn't really a criticism of the studies (they're probably doing the best they can, and achieving good results on those is obviously a prerequisite), but rather of how much we can extrapolate from them and how much weight we can put in positive results currently. No matter how perfect their results, I'd totally expect people to be reasonably skeptical of their extrapolability (if that's a word) in the beginning.