The undersized sizer die, or "the U die", is like any other variable, you have to test with and without. If you don't, it's a waste of time and we're just guessing. Some people use them for a specific purpose where they have had sizer issues and report success. Some have tried them and found them unnecessary for their application. Some have tested them and found little to no difference.
You need a reason to try them, which is poor neck tension traced to the sizer being too large. It has been properly suggested to test the other variables and making sure it is indeed a sizer issue before going that route.
I have tried the U die in two different applications, finding that they didn't really solve the issue, and ended up solving it another way. YMMV, you just have to test things carefully. Others here have found them to be the cure in their application (Their dies, their press, their bullets, etc) and have happily announced the problem solved.
IMHO, they are not going to be needed the vast majority of the time. The die makers, bullet makers, etc, really do a fine job putting out quality products that work the vast majority of the time. They also have some of the best customer service out there.
Neck tension is case wall thickness, sizer die inner dimensions, and expander outer dimensions. If we lack proper neck tension, one or more of those things is guilty. It's our job to narrow it down and be sure before we buy different dies etc.
