UBI is a fantastic idea in theory. It is more efficient in terms of capital and human resources than the current system, probably by an order of magnitude or more. However, it does suffer one deep flaw that I haven't seen anyone promoting UBI address.
How do we ensure that UBI doesn't become weaponized against the people it's supposed to be helping?
If UBI is passed it becomes the single most important issue to the majority of voters. Someone running on a platform of 'more money' will (even if it's economically a bad idea) likely win elections based on it. If there is an economic need to decrease the UBI payments it will be the death knell of political careers.
But the biggest concern I have is this:
We've already seen big legislation like the Affordable Care Act come under attack and eventually get essentially removed (in that case overwritten by the American Health Care Act on Mar 4, 2017). What would the damage be to the people who come to rely on UBI as part of the general economic system, only to have it rapidly reduced or removed entirely?