Monetization is a huge deal, because once you start monetizing spending as regular policy, you undermine both confidence in the currency and confidence in the government debt; one of the whole reasons that governments have made monetary policy separate from fiscal policy through central banking is precisely to preserve that trust.
While UBI itself can be managed so as not result in runaway inflation (and, unless you deliberately build a positive feedback loop into the level setting, should be self-limiting to avoid that in the long term, though the wrong initial setting could have high short-term inflation), monetizing the UBI, like monetizing government debt, is a great way to produce high average inflation, destroy government creditworthiness, and increase volatility in the currency.
If you want to redistribute downward, taxing the wealthy directly is a lot more sensible than monetizing spending. And, in any case, none of the people that are opposed to taxing the rich are going to support monetization, and plenty of people that do support taxing the rich are going to oppose monetization, so it doesn't even make sense as a way of getting around the political difficulties of tax-supported UBI.