I was only reading last night someone in Florida who lost their FTTP twice, and it took out cell service both times too to his property.
It seems they have a problem with drunk drivers hitting poles and taking out the fibre and the theory was that the same poles also feed his local cell site, so both would fail. If he went outside his property, cell service would resume, presumably as he moved into range of a different tower.
So yeah, I think mobile backup is always going to be more practical, as there is at least a chance it wont be the same fibre route. Makes me a little sad as mobile backup, at least on pfSense, is kinda sketchy because packet loss can cause the whole firewall to restart, even if nothing is using it at the time. The alternative being to disable gateway action, which then means if it fails for some reason it wont automatically try to re-establish the gateway, which kinda sucks too.
I assume the Firebricks are more intelligent somehow, seeing as Weaver has already had a solid experience with mobile backup.