I just moved over to the G2 from the S4 (ridiculously locked down), and so far it has been great. I've noticed the screen flicker when it's under ~40% brightness, and it was an issue that was documented in this thread:
It seems it is at the kernel level. I've been snooping around, but it's a needle in a haystack when searching rom threads, so I'll just ask. Does anyone personally know if any custom roms (preferably the latest stock 4.4.2 build) have a modded kernel that fixes this? It's the one thing that is pretty annoying. Otherwise, getting rid of laggy touchwiz has been great.