It works with U-boot web ui 
I think from what I readed in various forks of OpenWrt core developers and contributers, it was primarily a issue with the motorcom drivers the 10gb ethernet port and sfp, the flint 4 has two separate internal switches one from motocom, initially there was some talk of support for motorcom by upstream linux but this version was different.
Then someone posted a link to the leaked source and license, I don't know if they worked on this driver source or the now released GL-iNet drivers (between you and me I don't think this is that important).
Now GL-iNet is releasing their drivers, but there is also some porting being done either way support for OpenWrt is almost done.
GL-iNet repo:
Blogic repo one of the core devs (please see the PR by shi):
The topic by @shi05275
And the OpenWrt forum discussions:
IMO, I won't worry too much I have been testing some self compiled builds and the ethernet ports work, there only need to be some more testing, like the hardware macs etc, possible hardware offloading, for me the reset logic still failed for me.
^in short: flashing should just work as intended