Brian's answer is pretty much on target with my experience, if it helps.
In INTENT, I thinkj the "Heroic" scale is meant to be easier to see as a unique Player-Character personality (hero) on an RPG tabletop, with tthe "Heroic" features exaggerated to make key features easier to see at a glance - especially things like fancy weapons, the curves and "assets" of female characters, the muscles of male characters, and most notably here, the face and eyes.
To the extent that the face and eyes are bigger and easier to see, it's going to be easier to paint fiddly details like lips, cheek blush, irises, pupils, reflections and highlights, and so on - I bet it's far more common for painters to go into really fine detail for a one-off heroic RPG character than to try to do that for each and ever member of a large historical army!
Conversions and the like might be considered easier for heroic fantasy gaming, largely because of the sheer number and variety of heroic fantasy miniatures and bits to choose from: if you want to build a very specific heroic fantasy character, you've got a lot of material to choose from, and the size of wrists, weapons, ankles, and the like are probably more forgiving than the more delicate features of historical figures.
Getting just the right unique costume, magic sword, and such for each character generally isn't going to be a big deal for most rank-and-file historical figures in common eras/subjects, but if you're converting a few boxes of historical figures for a specialized subject and need a bunch of hard-to-find equipment, helmets, backpacks, or whatever - especially if you need those uniform pockets and pouches and collars and webbing JUST right for historical accuracy - you're probablyin for a challenge compared to the more loose and broad-strokes one-off heroic fantasy hero..
I've collected a LOT of spare bits, and in my experience some of the worst offenders for kitbashing between historical and heroic models tend to be oversized heroic heads on historical bodies, and relatively delicate historical arms on heroic bodies: these can look pretty weird even at "tabletop distance"!
That seems to be more true of kitbashing between Wargames Atlantic and its competitors, than kitbashing between two Wargames Atlantic kits, even historical vs. heroic, though as noted by others about the Grognards, that's not going to be universally true from WA vs. WA kitbashing.
I've never paid enough attention to confirm this guess, but I'll go out on a limb and guess that the most exaggerated "Heroic" Wargames Atlantic kits (and thus the ones that might be the most difficult to match with historical bits) are probably going to be the earlier kits produced by WA (and the Grognards certainly seem to fit that theme, if my guess is right!)
In terms of heroic bits sourced from WA's competitors, some of the weirdest heroic proportions I've seen have come from Games Workshop (especially older kits - the figures from the '80s and '90s can look especially weird compared to WA historicals), Frostgrage, Oathmark, and Dead Man's Hand, (There can be a lot of neat bits to work with in these kits, and they generally mix-and-match very well between heroic figures and kits, and it's possible to mix-and-match some stuff between e.g. the WWII French Reistance and Dead Man's Hand or th elike, but in a lot of cases these mixes look a bit odd on historical figures if you're not careful.) I've yet to handle a WA heroic kit that is nearly as exaggerated as any of these! (To be fair, I've not seen the Grognards first-hand, so they might be an exception, and I've never actually assembled the WA skeletons, only broke them up for scenic bits, so they might be an exception and I don't realize it - it's tough to judge skeletons anyway.)
At the end of the day, I've NOT handled the Age of Reason kits, so I can only guess, but I think that WA's newer "heroic" stuff is not as exaggerated as it used to be, and it looks to me like at least the newer WA "heroic" and historical stuff is probably close enough to take a chance on.
It sounds like a fun project, I thought about grabbing a box of the Minute Men to part out as fantasy bits myself, adding elf heads and "gothing" them up a bit into Unseelie elves, and the only thing stopping me right now is uncertain funding and I'm still recovering from going blind a couple months ago - if you take a chance on it, please do post pictures!