OK - I'll bite. I normally don't get drawn into these discussions but here is my view.
Universal Background Checks are ostensibly proposed to prevent criminals or someone who otherwise couldn't buy a firearm from doing so via a private party transaction. (Gun show parking lots, chat rooms, classifieds, etc.)
If I believed that was end game, I would have minimal concern over it. What does concern me is that I do not think the gun control lobby simply wants to prevent criminals from buying weapons.
A large percentage of that lobby want to stop or severely handicap the ability for anyone to buy or own a weapon. UBCs would be a 1st step and they would not stop there. There would be no appeasing that appetite.
Given an inch, I believe they would want to take a mile, and that could spill over into numerous areas of infringement such as loaning a gun on a hunting trip, passing them down to an heir, etc.
A UBC would tie a serial number on a 4473 to the buyer and the seller - just as it does through your FFL. If required for a period of decades, it would in effect create a pseudo registry for any owner who followed the law - and therefore sets the stage for discussions on confiscation.
It simply won't stop, and compromise on something like a UBC won't lead to finding a common ground on which there could be agreement and a "truce".