Having said that (preceding post to this one), there are some limits I ran into with UniFi but they'll be unusual and inapplicable for most people at home:
1) I also have a Fiber Channel SAN that runs between a couple of PC's and an LTO-8 Tape Drive. On more advanced networking equipment you can bridge Fiber Channel over Ethernet (FCoE) directly, and then run your Fiber Channel traffic next to your Fiber Ethernet traffic over the same set of optical fibers and switches. UniFi can't do this. However, equipment that CAN do this - like Brocade - is 10 times the price of UniFi.
So I instead just keep my Fiber Channel stuff on a separate HPE switch and I run it on different physical fibers next to the Ethernet fibers. Thankfully when my installer originally ran my fibers, he used a roll of OM3 cable that had 6 strands of fiber (3 pairs), so all my outlets in the house now have 3x LC-LC pairs. So top pair is now Ethernet, middle pair is Fiber Channel. C'est la vie.
2) Since Frontier gives you a fiber line to the home, and the UniFi Gateway takes an SFP+ module, you should just be able to run the physical Frontier fiber right up to the UniFi gateway, then use a SFP GPON ONT, plug the fiber into the SFP module, and plug it into the UniFi gateway. In theory. What happens in practice though is that Frontier runs their fiber to an outside Frontier ONT box, and from there they run CAT6 into your UniFi Gateway. I know FS sells compatible GPON ONT SFP modules. However, trying to ask anybody in Frontier about provision a SFP based GPON ONT is like trying to explain astrophysics to a goldfish. I just gave up.
XFinity Gigabit Pro on the other hand drops fiber into your house (850nm MMF - identical to the stuff I run in my post above), and you connect it via a SFP+ transceiver directly to the UniFi Gateway.
Fully sanctioned by XFinity. But XFinity sales refuses to acknowledge the existence of XFinity Gigabit Pro, even though it's still on their site and they literally came to my door and advertised it.
So even though the UniFi Security Gateway allows for 2 direct SFP+ fiber-based ISP connections, I know of no ISP that actually can provide direct fiber. I think UniFi also gave up on it since the new Dream Machine Pro only has 1x SFP+ WAN connector instead of 2 like the old USG. Their solution for a failover ISP is now via RJ45 only.
UniFi also now has their own
LTE-based failover device that they sell, which is just a POE device that you plug into any UniFi switch on your network, and that they then tunnel back to the USG or Dream Machine Pro for security. So maybe UniFi does not think a wired ISP failover is that important anymore.
3) Their ISP failover works great for inbound failover. It does NOT work great for outbound failover. I would expect if there is a ISP failover, that they will automatically re-register DDNS records with the failed over public IP address - it doesn't do that. Having said that, that's only really useful if your primary ISP goes down for days at a time rather than just a few hours. Otherwise by the time you have DNS propagation of your secondary, it already switched back to the primary.