I have a very strange issue when I enable FastRoaming on 5GHZ on a a particular device (Netgear R8000 (BCM4709)). On other Linksys Routers 3200ACM and on Xaiomi fast roaming is working fine. Basically when I enable the option I wouldnt be able to authenticate to the SSID.
Just like wrt3200acm with its mwlwifi drivers, brcmfmac as used in the r8000 is also a fullmac driver. This means pretty much all of the connection handling is done in its proprietary firmware blob running on the wireless chipset - out of reach of driver and kernel, as a result there generally isn't a whole lot that can be fixed without Broadcom's/ Cypress'/ Infineon's participation.