I wouldn't like to see the FOG engine used for anything more recent than Renaissance. There are two reasons for this opinion of mine.
First FOG does not model the actions of a few soldiers, rather the bigger formations. Once hand-held gunpowder weapons were available en-masse, many armies used small detachments and even individuals in a sniping and harrassing role and sometimes their effect was disproportionate to their numbers. I think you'd need to model that in a gunpowder ruleset.
Secondly, if all periods were essentially FOG, wouldn't there be a great danger of loosing the variety that we have as wargamers to try out different periods with their own distinctive flavour? Would it not entail a "boredom" factor which is alleged to have hastened the decline of DBM?
Anyway, I know the FOG authors have a great deal of ancients knowledge between them, but I am not aware if any of them have a great knowledge of say, WW11?
Keep FOGing in pre-gunpowder I say:-)
Regards
Dave M