The issue with the Factory Crimp Die is that the carbide ring in the die will resize the brass "to ensure that it is within SAAMI specifications" for diameter. For jacketed bullets, it usually works fine. (Actually, I am finding that many factory jacketed bullets are a little under-diameter in .45 ACP.)
But, cast or swaged lead bullets are usually made about 0.001" larger in diameter than jacketed bullets are SUPPOSED to be made. So, the carbide ring MAY slightly resize the cases once the bullet is seated and slightly expands the case. The problem with that is the lead is NOT springy, while the brass is, so the resizing with the bullet inside the case will tend to make the bullet loose in the case. For autoloaders, that can be a safety issue. When the bullet is fed from the magazine, it hits the feed ramp with enough force to push the bullet farther into the case IF the case does not grip the bullet tight enough. If the bullet gets pushed deeper, pressure goes up. It CAN go up to dangerous levels, depending on the particular powder, bullet, etc.
So, you ALWAYS want to be sure that set-back won't be a problem for your handloads. It is not just a matter of the Lee FCD; there are other issues like too-thin case walls that can lead to the same problem. SO, you need to test the rounds that you load to be sure that the bullets are gripped tightly enough.
For the FCD, if you FEEL the cabide ring sizing the bullet when you withdraw the cartridge from the die, then you really need to check that the bullets are not too loose.
Increasing case grip on the bullet is NOT a matter of making a heavier crimp, which can actually loosen the bullet for the same reason described above (in cartridges that head-space on the case mouth so that they must use a taper crimp instead of a roll crimp). You need dies that first, size the case small enough, then do not expand it too much, and finally, don't over-crimp or post-size the bullet inside the case.