My assumption is that UBI is a significant transfer of wealth from the richest to the rest. Isn't that the whole point? Exactly how to structure the taxes that pay for it is naturally a key question.

Given that, it's pretty safe to assume markets that cater exclusively to the ultra-wealthy will be harmed by reduced demand as their customer base shrinks. Higher tax rates will also exert downward pressure ob stock values as companies make less profit and investors tighten their belts. (Especially if there's a wealth tax.)