As previously mentioned by @frankis, I can confirm the 5Ghz radio is unstable (tested with Archer C6 v3.2, same hardware as A6 v3).
If two devices make intensive use of Wifi, the 5GHz WLAN simply dies. OpenWRT shows radio as active, but the SSID stops broadcasting and clients loose connection.
Restarting the radio does not help. Only a reboot solves the problem.
The issue can be easily reproduced with two WLAN clients running IPERF3 to two different servers at the same time. Country Code was not changed and set to "driver default", this does not solve the problem.
Regular web browsing does not cause problems. Only intense use of Wifi by two clients cause the problem to appear.
Issue tested and reproduced on SNAPSHOT r17419-364bd887a1.
EDIT: I failed to mention I tested with "IPERF3 -R -P4" in both clients. If I reduce the number of parallel streams to "P3" or less the problem does not happen. But if I run both clients with "-R -P4" simultaneously the OpenWRT 5Ghz radio almost immediately dies and only a reboot restores it.