I was once an avowed advocate of the UCB but no longer am. I have a half dozen or more of them gathering dust in various places now -- but nary a one in a reloading press.
They cost $30, plus tax or shipping. For that you can buy more than 15 powder bushings.
A new one usually takes an hour or more of sanding, honing, testing and more sanding, honing, testing to get the thing moving smoothly through the channel. I've never had a new one yet that didn't require that process, which is a PITA.
There is a chart that usually comes with a new UCB. It's about as accurate as a powder bushing chart, which is to say it'll get you in the ballpark but you've got to tinker and weigh charges to get the right setting.
The real-world reality is this: Shotgun shells have a fairly wide range of acceptable charges and there's usually a bushing that will get you acceptably near the velocity you want.
Anyone who thinks he'll get more accurate shot or powder charges is kidding himself. The UCB is still a volumetric measure and suffers from the same inconsistency as charge bar cavities and powder bushings.
BUT... There are some recipes and powders for which the bushing jump is simply too much and in that situation the UCB is a good gadget to have on hand.
Another point in its favor is the ability to adjust shot charges without having to buy another bar for from $9 to $14.
At a minimum, I have at least one MEC progressive and one MEC single-stage reloader for each of the gauges and .410 bore.
They all have standard charge bars and standard bushings, and I get exactly the shot and powder I want from each.
But maybe I'll take Rem 3200's suggestion, hunt them down and use them as paperweights.