The LG G3 experienced two problems that lead to non-functionality and eventually death. First was a failure to recognize the SIM card, which can sometimes be repaired with a new sim card tray. The second was the flicker and fade screen failure. Probably actually a chip or solder failure in the video subsystem.
In my opinion, the common cause of these is faulty soldering or faulty solder. Perhaps they have not adequately perfected RoHS lead-free solder profiles. Lead-free solder is a disaster overall and is leading to short device lives and way more e-waste. The lead-free solder can't predictably handle thermal cycling well, quickly ages, cracks, and can generate tin whiskers.