In fact there is very little difference in available RAM after boot between AC68U and AC86U. With AiProtection and Adaptive QoS both routers have about 120-140MB RAM left. AC86U may have 2x RAM, but the software it is running is also using more RAM.
I am not talking about "plain" router situation. I was specifically referring to the situation when you have several addons running on AC68, especially "heavy" ones like Diversion and Skynet.