Originally Posted By Primos:
Not so much the sight, but the railed gas blocks. They're often aluminum, clamp on, and are short enough to prevent you from folding your front sight all the way down. So you have to flip it around and fold it forward, which then sticks out forward of your gas block. You're somewhat limited in options for flip sights meant for gas blocks, too, although Troy just released their version.
I would go with a built-in if you want to have a flip-up gas block front sight. I had a YHM and it worked fine.
Personally I prefer a regular FSB over anything, but YMMV.