I had 2 previous gen Airport Extreme Base Stations running 2.4 and 5GHz bands on channels that aren't populated which worked perfectly bet all my Mac's and iOS based device. I upgraded my 2x 6th Gen Airport Extreme Base Stations with "ac" (our Mac's and iOS devices are new and use "ac" radio protocol). First few months were great, since 10.9 I've had sporadic speeds, drops, overall inconsistencies. My 2013 Mac Mini HTPC works perfectly directly connected to my modem. I've completely reset both routers, changed the WiFi names and passwords, removed and reset my Mac's saved network settings, yet still have experience random drops, etc.
God I miss Bertrand Serlet and 10.6.