At least in my case I flashed 6.0.21 to the v3, which seemed to boot, I then flashed openwrt, and now the device seems to be pretty bricked. The white LED comes on immediately and just stays on, and I can't seem to interact in any way (I've waited over an hour to see if it was just taking a while or something), even trying to get into the recovery mode doesn't work. Going to open it up when I get a chance to, to see if I can glean anything / fix it somehow with access to the serial pins.
(more details of exactly what happened I had written about here Unifi 6 LR Issues)