Thanks Dave, your reply is very informative. I definitely understand that FreeBSD is somewhat less sophisticated than Linux when it comes to this since I've read through a few of the bugs and other messages you've posted regarding its implementation. However the Unifi routers run on Linux, and they still require the user to specify the ISP's service bandwidth. I think this paragraph from your reply may be the crux of it:
> Further most ISPs use a non-native rate for their customer interfaces,
> either using a policer or FIFO shaper and thus the rise of combatting
> that with shaping via fq_codel to slightly below the ISPs' rate to
> move the bottleneck to your own hardware.
If the router is able to keep pushing bytes over the ISP's link even in excess of the allowed bandwidth, then the local router's packets probably aren't sitting in their queues long enough to be dropped by fq-codel. It sounds like proper traffic shaping needs to happen wherever the bottleneck is (I suppose this should have been obvious to me), which in the case of a 300 Mbp/s connection over GPON is somewhere at the ISP. I guess if the ISP does proper traffic shaping the end user shouldn't need to do anything.
Assuming I didn't say anything inaccurate in my reply, I think I understand why setting a bandwidth limit is necessary to make fq-codel work properly on a router with a less than line speed connection from the ISP.
Thank you!