My setup:
->RT-AC86U (Primary node)
->RT-AC68U (Repeater mode, connected through WIFI on 5GHZ band)
->APANAGE 11-Port USB 3.0 Hub
--> 7 drives in total connected to the USB 3.0 port on the RT-AC68U (File server)
--> USB3.0 port is set to USB 2.0 on the RT-AC68U in order for my USB hub to be detected properly
Summary of my experience with the lastest 3 official firmware from ASUS:
1) 3.0.0.4.384.81049
This firmware was by far the worst and with this FW loaded on both of my routers, I was unable to get the RT-AC68U to connect to the RT-AC86U in wireless repeater mode. I couldn't really test anything, and shortly afterward I rolled back to the previous firmware.
2) 3.0.0.4.384.81351
The recently released FW allowed my RT-AC68U to successfully connect to my RT-AC86U and everything appeared to be working; however, this FW caused major connectivity issues that resulted in very strange delays when trying to load a webpage, stream a video, or transfer files between my external USB hard drives. File transfers from one USB hard drive to another would transfer data for about 10-15 seconds, and then would just stop and hang until the file transfer would simply fail. Although the routers remained connected, there clearly was frequent packet loss or WIFI drops that caused any file transfer to simply stop working.
I tried statically setting the control channel for both the 5GHZ and 2.4GHZ band on my RT-AC86U (cannot set this setting from the router in repeater mode). This change had one interesting effect, it stopped the WLCEVENTD messages from appearing in the main logs, but this change did not resolve the strange delays or help my file transfers. Clearly this firmware is a step in the right direction and if you do not have a similar setup to mine, then you may not notice these issues. As I use my RT-AC68U as a file server, I cannot use the AiMesh mode as this mode disables all access to the management page on any node beside the primary node.
Since my RT-AC68U remains connected to my RT-AC86U with this FW, I assumed all was well, but there must be a lot of very brief disconnects that do not show up in the logs.
3) 3.0.0.4.384.45717
This version is by far the most stable and best-performing FW that ASUS has released. I get no wireless disconnects, no WLCEVENTD messages, even if I don't bother to statically set the control channels. Accessing websites, streaming videos, or performing any type of file transfer works without issue.
I wish the latest FW's worked as well as this one, but I just can't envision ASUS fixing the issues I am observing anytime soon. If the latest firmware works well for anyone else then stick with it, but for me, FW 45717 is by far the best and most stable FW.