The problem I have with UBI right now is actually how it is often framed by liberals, which I think reveals its weaknesses as a policy at this moment. I temperature check some liberal forums on a regular basis, and I saw that in response to this Sanders news some liberals were upset that he wasn't going for a UBI because they felt that the JG would just create busy work in the face of automation. What is interesting about this to me is that, they didn't question capitalist property relations at all. The UBI didn't inherently drive them further left, though I consider that an inevitability of the suggestion as a cure for major automation. If you don't change capitalist property relations post-automation, but instead simply give people a monthly allowance, then the capitalist class is almost indomitable in its position from any reasonable angle. Normally a justification for their status is meritocracy, but without a means for the vast majority of the population to rise in a company since jobs don't exist anymore, the capitalist becomes a lord of their capital, and the capital can only change hands based on their will. This seems to be clearly unjust, even to a moderate liberal, maybe even to a conservative. I think this would have to bring about a questioning of what entitles one person or another to ownership of production when everybody else is dependent on it but totally disenfranchised from the process of production outside of their ability to vote.
Which is to say, I think UBI right now is jumping the gun for something that simply hasn't happened yet to a significant enough degree to make it the center of policy. In fact, I'd think a UBI, when it was needed, would be short lived because a cultural awareness would arise about what I just said, and eventually the terms of ownership of capital would become attacked by the state on behalf of voters. A JG, on the other hand, paired with normal welfare type benefits for those who can't work, allows the state to control the reserve army of labor right now and set the standard which capital has to measure up to, it logically creates the conditions for larger economic planning to occur at the margins in order to make the most productive use of the unemployed, expanding social goods and welfare, and it guarantees benefits and income to anybody who can work. If automation makes work so obsolete during the time of the JG that 3 people are paid to stand on each street corner and watch children cross the road, or dig proverbial holes, then we already effectively have a UBI with busywork, and we can move into a full UBI/full nationalization of the economy as it becomes apparent to everybody that the system has become absurd.
So no harm no foul, and we'd have more to gain right now from a JG both in terms of diffusing inequality and attacking upper class power than UBI, which seems like it is primarily trying to get ahead of an automation crisis that really isn't much of a crisis with JG anyways, just a kind of temporary headache for everyone who has to do busywork until the program is changed.