I am new around here but this thread has been helpful and I have been somewhat successful with AIMesh this week so I thought I would share what has worked and what hasn't.
I currently have an RT-AC5300 running the AIMesh Beta as well as (2) RT-AC3100 running the release AIMesh firmwares.
Initially I had the 3 setup as the RT-AC5300 as my main router and the (2) RT-AC3100's running as AP's connected to the RT-AC5300 via Cat6. While this setup worked, it never worked well with a merged SSID (2.4ghz clients sometimes wouldn't connect) and separating the SSID's was just too much with 3 routers.
Also on my network are multiple Netgear GS108 managed switches handling bonded LACP LAG connections between my workstations and servers, as well as a Zigbee/Z-Wave smart home setup with over 110 devices. Needless to say, there is a lot going on in the 2.4ghz spectrum, but channels have been isolated and everything works relatively well.
On Monday, after updating all the firmwares on the RT-AC5300 and RT-A3100's, I reconfigured the RT-AC5300 as my main router and reconfigured it as it was with all the port forwarding, DDNS, etc, etc, etc before attempting to setup AIMesh. Once I was ready to configure the nodes, I did factory defaults on both RT-AC3100's and they both were found and paired to the RT-AC5300. I then was able to move them to their locations and set them up with ethernet backhauls to the RT-AC5300. Everything seemed to work fine for about 24hrs, but after returning from work the next day I found both nodes had dropped off and couldn't be found again.
After reading many of the comments here echoing the same issues, I decided to try a different approach. I grabbed my old RT-N66U out of the closet, removed it's antenna and installed the latest version of Merlin on it to be used as just a gateway router with all my port forwarding, DDNS, etc, etc, etc configured on it. Connected to the RT-N66U via cat6 are my GS108 switches and the RT-AC5300. Once all that was setup, I then configured the RT-AC5300 as a AIMesh AP Router, only following the guided setup to assign the SSID, security and static IP. I then reset the RT-AC3100's and they paired right up with the RT-AC5300 as nodes. From there I moved them to their locations with Cat6 backhauls plugged directly into the RT-AC5300.
So my config looks like: ISP Modem -> RT-N66U (No Wifi) -> RT-AC5300 (AIMesh AP Mode) ->(2)RT-AC3100s (AIMesh Nodes)
For the past 3 days, all has been working perfectly, both 2.4ghz and 5ghz clients connect and move seamlessly throughout the house
Asus has a long way to go to make this user friendly, but when it works, it's pretty good.