I don’t know what “too hand wavy” means.
Example:
A: The reason it all works is <waves hands>.
B: I have no idea what <waves hands> means.
A: Exactly.
https://en.wikipedia.org/wiki/Hand-waving
By that, I think they meant they don’t want to enumerate the problems with UBI, of which there are many.
Example:
A: The reason it all works is <waves hands>.
B: I have no idea what <waves hands> means.
A: Exactly.