I stumbled on Hill Cantons' posts about AD&D's implicit post-apocalypse setting again recently. I conclude that one of the difficulties I have had with ACKS is that my settings want to be post-apocalyptic, lawless wastes dotted with city-states ruled by tyrannical despots. ACKS' implied setting, however, is of an empire in decline, where order has yet to break down. The apocalypse is a thing that happens in play, via domain pillaging and Cataclysm rituals.
I'm sure ACKS' demographic guidelines can handle starting from ruins, but I may need to push them a little. The real troubles in play are likely (and have been) market class and making any sense of "how does anyone survive here", which heavily-armed patrols on the civilized encounter tables answer historically.
Thinking about it further, Simplified Domains are actually perfect for this sort of world. They reflect a very centralized, "everyone lives near a walled settlement that you can flee to" settlement pattern. They're self-contained, with no "radius of civilization" projected around large markets (I mean, there is a "radius of vassals", but that's easier to deal with and leads to fewer incongruities, like "well this trackless wasteland is technically within 25 miles of town, so I guess it's civilized and there are peasants here"). They have very clear garrison numbers and suggested ruler levels, which though lower than what AD&D would suggest are workable. They resort to feudalism only when administratively necessary, which is fine by me because I tend to associate feudalism with high-medieval rather than post-apocalypse. Feudalism is order; order has collapsed, now all is despots, with one layer of treacherous, non-hereditary crony-"counts" beneath them. I think if I worked up stats for independent counties and duchies and tweaked the troop types a bit (towards "mobs of light infantry in Mad Max-style black leather with spikes, armed with mismatched Iron Heroes-looking weapons and supported by cavalry on wasteland beasts, with city-states fielding rangers and paladin-armigers" instead of "pikes and cavalry"), I'd be in good shape to throw a bunch of city-states and bandit camps on a map and start running.
Actually Iron Heroes is also a pretty good basis for the general aesthetic I want in a game, now that I think about it, though I could take or leave dangerous magic... maybe steal DCC's casting rules? Hell, you people keep asking me about patrons for Sorcerers, those DCC guys have threads and whole books of 'em just waiting for conversion. Anyway.
Looking at it briefly, I'm pretty sure an independent simple duchy would actually have a class III market under current assumptions, which is adequate for most mid-level operations. I also like that the max level from domain XP for dukes is 13th, which means that they're about the top of the food chain. It is also true that an independent simple county would have a class IV market, which is perfect as a base for low-level adventuring.
One additional advantage of such a setting for ACKS is that political legitimacy is relatively easy to come by, for PCs. You don't need to worry about heredity, swearing fealty, de jure claims, and justified war; political power flows from the tip of your sword, by which you maintain internal order and deter external threats.