I support them because most gun sales already have them, they're usually performed within minutes and add no significant administrative burden, time or expense to the transaction, and it mostly eliminates the need to define who is a "dealer" under the BATFE rules.
However, UBC's are still a solution in search of a problem. They would not have stopped any of the mass shooting over the last decade or two, nor the vast majority of all other gun crime, accidents, or suicide. Calls for UBC's in response to such shootings are totally disingenuous, and little more than "not letting a crisis go to waste." That's why proposed UBC legislation is almost always combined with other gun control wish list items like "assault weapon" bans and magazine limits, and similarly the reason why such legislation is rejected out of hand.
Gun rights proponent are not stupid. We recognize a strategy of incrementalism by gun control advocates. Just as abortion rights advocates always (and quite correctly) strenuously oppose all seeming minor and ancillary abortion "safety" regulations because they know the intent of the sponsors and their ultimate goals, it's entirely unsurprising that many gun right advocates react the same way to "gun safety" proposals, particularly when advocated by the same people who routinely and historically have demanded far greater restrictions on firearms, including both President Obama and Secretary Clinton and their support for the Australian system as a model for American gun control, i.e., prohibition and confiscation.