I’m not a fan of WHFB, but I believe was a fantasy system before it became a historical system (WHAB). But it’s tied to a specific fantasy world, which, though obviously inspired by the original D&D, isn’t D&D.
Part of the problem with mass fantasy battles in the D&D world is that D&D was itself created as a medieval world with magic layered on top. Thing is, magic is like technology— if you introduce it to a society, that society is going to radically change. So if you have a world where individuals can lob magically created fireballs to explode into massive bodies of troops, you’re going to wind up with significant changes in the military tactics of that world— not unlike what happened in our own world with the advent of tanks, machine guns, and precision bombs. No longer will you want tightly spaced ranks, side by side, but very loose, open units where the effectiveness of a fireball is mitigated because the troops are too spread apart to take out more than a few at a time. And that’s just the changes brought about by one spell! AFAIK, no current fantasy battle game attempts to recreate tactics “from the spellbook up.” But then, that’s not generally what people want to imagine, anyway. They want the Charge of the Rohirrim from LotR, and they want fireballs flinging about left and right. Realism can take a hike.
Personally, I like the Warmaster game, also from GW, though, alas, OOP. It does do mass medieval combat well, with the effects of magic largely simplified and abstracted. (For those with a Tolkien bent, there was also an LotR-style version named The Battle of Five Armies, which offered the forces from The Hobbit (the novel, not the awful movies), and modified the magic rules to that setting as well. Like Epic40K, the game has a “grand tactical” scope, using 10mm figures mounted en masse on rectangular bases (“stands”), with three stands forming a “unit”. Most battles involve 12-15 units per side, though the game can handle more. Great game, far superior to WHFB. For awhile the rules for WM were available as free PDFs online from the author (Rick Priestley), but I don’t know if that’s still the case.
For those reluctant to invest in miniatures, Warmaster can be played with cardboard markers instead of figures, and there are lots of fan-created markers out there in the Web as well.