Well, they tend to do two negative things to the pistol: any material that comes off the feed ramp of the barrel, even if they leave the angle the same, is going to increase the amount of case head that is unsupported. And most people want to put a little shallower angle on the ramp to make it feed slicker - which further reduces the amount of case head support. The other thing that amateurs do is to put a slight hump in the angle of the feed ramp. It is hard not to wind up with a slighthump when using a Dremel tool. A perfectly flat surface feeds much better. And a slight concave is a little better yet - particularly on a fully ramped and supported barrel.

Most folks would be better off to just let the bullet noses do their polishing. I havea shoebox full of ruined barrels that were the result of a DIY feed ramp job. Some of these barrels caused case ruptures that injured the shooter. Polishing a feed ramp properly is a lot harder than it looks . . .