Back when the 777 was shiny and a novelty, used to use the FPV all day long including for unreliable airspeed events, which made for low-stress procedure. about 12 years after it was shiny, Boeing came out and suggested not to use the FPV as a back up for unreliable airspeed, as somewhere along the line, the FPV has some input that could give invalid input. Looking over the architecture, no obvious reason why that would be the case jumped out. Never saw a fault that took it out, but then they nearly never faulted anyway except in the sim, and sims simulate sims not aircraft. The FPV when there is worth its weight in Guinness.
The relevance of the above boring story is merely independent synthetic information can be had, but the more inputs that are added to make it feature-rich, the more failure modes exist and usually take time to come out of the woodwork. For 199, get a Dual or a bit more for a Stratus, and have ADSB IN, syn head down vision, backup attitude without the need of an emergency battery, terrain alerting, runway entry alerting.... traffic alerting, moving map, weather overlay, etc... the software guys are reticent to add FPV which is an ongoing irritant. They have attitude from a stable MEMS AHRS, with MEMS rate gyros, they have flight path, from the GPS.... and yet we get ATT but no FPV. We get ROD and GS.... whoopee, give an FPV and get rid of the sink rate to declutter.... or not, but please give the FPV. FPV and syn vision.... life is grand. Foreflight, Garmin, please...
We operate on EDTO/ETOPS limits which are great, say 208 minuters or 180 or whatever, and we are protected from everything except for the loss of power to the flight instruments, in that case, the 2 hours that you get from your ISFD less the route distance (time) limit is less than zero. an iPad is a nice thing, so is the dozens of apps that give attitude from your android or i-fone.