The primary concept for using Fg instead of Ffg, I think, is to reduce the pressure generated by the charge going off. It is considerably less, which could be an issue as bore size and powder charges increase.
A while back, I did a conparisson test on my 8 bore and found that with a full charge of 300 grains, with Fg I got 1,435 fps out of it, but with Ffg, it delivered 1,525 fps. The real surprising part of that test was that I tried it with Pyrodex Select RS and it gave me 1,610 fps. Wow! I really was not able to detect a difference in the recoil between the different powders.
Another issue with the Fg is it is dirtier than Ffg and leaves more residue.
My general rule of thumb is to use Fffg in pistols and rifles up to .50 caliber. From .54 caliber to .72, I use Ffg, and for anything larger than that, is Fg. Strangely enough, it is my understanding that some of the BPCR shooters are using Fg in their .45-110 and larger. Not sure why.