I am using Unifi 6 LR with OpenWrt for a while without a problem.
- For the first device flashing, I downloaded the image from the device page. This image doesn't contain Luci, and the LED light after the boot finishes is off.
- So I freaked out and tried to roll back with the TFTP method and an original Ubiquity image. I didn't do anything wrong, but there is no way to flash the device using TFTP after flashing OpenWrt. Gladly I did find out that I can ssh into the device where a standard OpenWrt is running. Afterward, I flashed stable 21.02.1 from the repository, and everything is OK.
- This device has two wifi devices: MediaTek MT7622 802.11bgn and MediaTek MT7915E 802.11nacax I am running the second 5ghz deice in ac mode - setting to ax causes randomly, after some reboots, a dysfunctional 5ghz showing bg (or something similar). Meanwhile, you get decent ax wifi speed, but it does break after some time/reboots.
I am looking forward that a future firmware release does fix the ax issue.
PS: Afterwards I did convert more of them to OpenWrt.