The 'PBR' system doesn't even use fresnel controls in this way because visual difference is fairly minor with exceptions (like precision for jewelry rendering, but those aren't done in UnrealEngine :- ) )
I mean, our eyes can't really distinguish between 60 or 65perc. reflectance which will occur at certain angle, which is what the complex fresnel describes. So for absolute physical corrected, and specific needs it's good to use,
but by all means, not at all necessity.
I create Chrome completely different for every application :- ) Sometimes I use intentionally super low reflective chrome to get contrast in room where there is none. It looks physically correct because it's still metal, it just isn't real chrome.
But who cares ?
The "inverted" way in PBR comes from Disney's paper where they defined certain range for non-metals (1.3-2.4 IOR ) and interpolated them to [0 - 1] values for artistic control, which default 0,5 being something like 1.52 IOR (default in Corona).
For metals, they interpolated something like 8-40 IOR into [0 - 1 ] again, with default somewhere in middle.
It's pretty smart solution because you will always work with physically correct reflectance, and everybody can tell if something is metal or not, or most of the time :- ) And default values reflect that. Going more complex than that is rarely benefitial. Good textures are far more important.