The real downside of Yang's plan is not including children in it which is odd because he talks a lot about a UBI funding the work of parents staying home with their kids.
The ideal scenario in my mind is a property tax, of some kind, that funds a UBI for children. Ideally, this would be at the federal level, but unfortunately property taxes have been deemed unconstitutional. This is likely true for wealth taxes as well.
But there is nothing to prevent states and cities for instituting a property tax -> UBI setup. It would no doubt help greatly with the problem of "buy properties and do nothing with them" aspect of rich foreign investors in some regions.
One of the things that Yang seems focused on is stuff that can actually be done. A wealth tax sounds nice, and he agrees with the spirit, but that is simply not a federal level option at this time. A constitutional amendment would be great to fix this, but that takes a long time and would require people to get out of survival mode to make this happen.
So Yang takes the practical approach of getting money in the hands of everyone now using a tax scheme that seems to be inoffensive to wealthy people. It probably is not, but the only way to really know is to do it.
As for the children, I am hoping this is also just a pragmatic thing. While it is hard to get this country to accept giving free money to other adults, it is even harder to imagine this country supporting giving money to "those people" with "lots of children". This is why schools are funded locally and why a UBI for children would need to be locally funded as well for now, particularly if the desire, as mine is, is to use land taxes.