I don't think this explicitly relates to the Merlin firmware, but that's what I'm running so I figured I'd mention it here. My RT-AC66Us have been unstable for months. Random reboots, incredible slowdowns to 1-2mbit/s every day (or even multiple times an hour if I was unlucky enough) requiring either a reboot or running restart_wireless over ssh, complete instability when trying to use CTF, etc. I didn't think my power adapters were faulty as I have 4 of these routers (only 2 in use...
RT-AC66U: increased stability after replacing power supply
RT-AC66U: increased stability after replacing power supply