Basic incomes can replace other kinds of welfare, thus simplifying the way welfare is dispensed. If you receive $ for food and $$ for housing through separate programs, why not simply receive $$$ through a single program instead? It'll be cheaper to administer, easier to conceptualize, and simplifies welfare paperwork etc...
It gives welfare recipients greater control over how they spend the money they receive. Requiring that certain sums of money are spent in a certain way restricts individual choice for the person the welfare is supposed to be helping. Irrespective of a welfare program's intentions, their awareness of an individual's specific situation and needs is dim enough that they typically are not the best judge for how the money should be spent. The obvious downside is that the individual who's receiving the welfare may not be any better a judge themselves. In particular, individuals whose predicament is partially caused by their own lack of judgement (e.g. an expensive drug addiction, or gambling) may find a basic income to be less supportive for their needs.
UBIs respect the monotonicity of earned income. This just means that if you earn more income (say through a raise, or a switch to a more profitable job), your overall income (after including the UBI) is guaranteed to increase. There is no povery trap of receding welfare.
There's a certain intangible unifying benefit to universal experiences. Knowing that everyone sitting around a table earns the same basic income lessens the divide among the participants. That said, many people may not find such benefits in other universal experiences, like jury duty, or paying taxes.
UBIS are exorbitantly expensive. A UBI in the United States paying out a measly $10,000 annually would require more than $3,000,000,000,000 (3 trillion USD) annually to fund.
UBIs decrease income inequality by raising the income floor. This has a similar benefit to universal experiences in that it lessens the divide between participants in the income scheme. Note that while income inequality is associated with a variety of negative factors, it should not be taken to be inherently a bad thing.
Basic income may discourage earned income by displacing it. Participants may work less or work for less if they have a guaranteed income that requires no effort on their part. This can present itself in a variety of ways:
Participants who can make do on the basic income through their own frugality may cease working altogether or, to a less extreme degree, may simply work less. For people who value leisure, this may present itself as a boon since it allows them to adjust their work-life balance towards the latter, however society functions because of work, and lower productivity means a generally lower standard of living above and beyond the lower standard of living a frugal participant accepts for themself.
Participants may be less willing to take low-paying jobs if their need for income is diminished. This can cause many low-paying jobs to go unfilled (causing the workflows that depend on these jobs to underperform), but may also force employers to raise wages in order to compensate to the benefit of the employee and the detriment of the employer.
Basic income encourages dependence on the state. This is true for all kinds of welfare, not just UBIs, but a UBI would be much larger than almost any other kind of welfare. Participants whose ratio of basic to other income is high enough may start to constitute a political class with the following characteristics:
Relative poverty, which is to say that the new class would be an underclass.
Dependency on UBI amounts. If the income amount is raised or lowered, it stands to significantly and intimately impact them. Politicians would have plenty of political incentives to make promises to raise it, and plenty of political disincentives to ever lower it even if it becomes unmaintainable.
Democratic states are supposed to derive their power from the governed, and this relationship experiences a priority inversion w.r.t. the new political class.
Having a UBI may allow for certain income-affecting regulations (like minimum wage) to be lifted. This makes the job market more flexibly by imposing fewer restrictions on it, and allows more jobs to be offered that wouldn't have been possible before because the value they provide was less than the minimum cost they have for an employer. Whether this is possible depends on how generous the UBI is; idealistic scenarios suppose that a UBI could aspire to offering livable wages (the same bar minimum wage tries for), but realistic scenarios always fall far short. Living off a basic income alone would likely amount to a kind of penury.
Cost of living varies participant-to-participant, and designing a UBI that takes this gracefully into account is an almost impossible task. People who live in cheap locations will get more freedom and flexibility out of a UBI than people who live in expensive locations, if the UBI is fixed between them. If the UBI is not fixed between them, it creates arbitrage opportunities by which people will be incentivized to move to where they can get more bang for their buck. Since people will generally always move along the gradient that most greatly benefits them, whoever foots the bill can expect the bill to rise.
Implementing a UBI dramatically alters the size, shape, and function of the economy. Any such significant change introduces vast new unknown unknowns into all economic reasoning, and with these unknowns, risk. The overall stability of the economy may be adversely affected, and the flux generated by starting a UBI (if it is not gradually ramped up from $0) may present a severe de-stabilitizing influence. Change is unsettling, and on such a universal scale, the ante is hard to under-estimate.