I really believe that most people using a U-die actually don't need it. I believe they are actually having issues with their Expander, and blame it on the Sizing die. That is, they simply don't understand how the dies work... they never stop to think about the process.
Lee was one of the first to offer a U-die and Lee is notorious for 1) having over-sized Expanders, and 2) capitalizing on the "rumor mill" to rev up sales.
To be fair, brass thickness does vary from brand to brand. In theory, that could affect the grip on the bullet. BUT, what if you could get the correct bullet clinching size you need by simply refining the Expander diameter with some emery cloth ? That 1) allows the user to dial in the exact grip they need and, 2) saves them the time and money of ordering, purchasing, fitting, etc a second die. Isn't it better (in every conceivable way) to simply run one die over ALL the brass, rather than having to sort brass by brand and then swap Sizing dies accordingly ?
I have nothing against Lee. I use some of their products. I salute them for bringing a line of American made, value-loaded,
highly ingenious products to our hobby which make entry for many new users even possible. But part of that Marketing ingenuity (I fear) is possibly being used to sell excess product.
Here's how the scam goes... the novice gets into reloading. Spends $30 on a Lee die set. Oops! An issue with the ammunition. Internet tells the novice to buy a U-die for another $18. Done. Later on, another issue pops up. Internet tells the novice they must have a FCD for another $35. Done.
But hold on... the novice just spent $83, when they could have had a set of dies from Redding or Hornady for $45 !! It also never occurs that brass was reloaded for 100 years without either accessory. At no time does anyone suggest that the novice stop, and simply
measure the dimensions of the brass at the root of the problem. This is extremely strange, because this same group of people are the ones measuring cartridge OAL to 4 decimal places and fretting about 0.001" of OAL variation.
The short story is that all these novices are running around looking for
solutions, when they should be looking for
understanding. Whether Lee is fanning the flames, or simply happy to sell a fix for the problem they may have created is unknown. All I know is what I've observed for the last 20 years.