Well, I remember the almost-unmanageable Encyclopeida Harnica or whatever. "more (badly-organized extremely pedantic minutiae-obsessed non-adventuring-related) trivia about peasant farm life!" is not necessarily the game advantage you seem to think it is.
The current state of how Harn articles are written is represented well by the fan made
Harn Pottage series on Lythia. At first Harn was tersely written by NRC, over the years the style has loosend up to include more details on individual personalities including adventure hooks.
A more recent example is the
Tashal Royal Amphitheater also fan made. It tersely sums of the details of how a medieval theater works as well as giving some adventuring hooks.
There isn't a single universal catholic and apostolic church that governs the whole of the civilized world in the name of one all-encompassing omnipotent, omniscient and omnipresent deity, is there?
No however from the viewpoint of individuals in most Harnic cultures it might as well be.
From Harnworld 1st edition Page 5THE GODS OF HARN
The inhabitants of Harn are pantheistic; they believe in teh existence of the ten major (and hundreds of minor) deitis., but most Harnians will worship only one of these.
From On Divinity Page 5
Polytheism & Belief
Hârnians believe in multiple gods, but most worship one (or sometimes two or three) at most. A follower of Laráni believes in Môrgath the same way that a medieval Christian believes in Satan. There is a considerable difference between belief and adherence.
Mix in the details of the religion that surrounds Larani, Peoni, and other dieties the result feel medieval authentic.
See, another downside
No you don't HAVE to read it, the medievalness of the Harnic religions is quite apparent from the core material. But is one is to nit pick the details are there explaing why the things are the way they are.
However there a more approachable work called
On Divinity a 22 page essay by N. Robin Crossby on how he view religion and how he applies to Harn and his campaign. Again it is nothing one has to have but there if one is interested.
Obviously, anyone who wants to run a medieval-authentic game is going to just HAVE to read at least a bit of (hopefully carefully organized and usable and easy to digest) material about the medieval world, medieval culture and medieval life. But if he's going to have to do that, why not learn about the REAL medieval world, instead of a made up place with funny accents on all the place names?
The REAL Medieval World spans from Charlemagne to Columbus at the very least. Which one is more real medieval? England? France? Germany? Poland? Spain? Italy? My view all are equally medieval although each have very different situation based on the reigon and specific time period. So an author writing a RPG targeting the medieval era has to be an editor and pick and choose which details to talk about and represent with mechanics. Some like the dominance of the church will be a common theme irregardless of region or time period. Other like manoralism and feudalism will be dependent on specifics.
I found doing historical roleplaying to be messy as source material and details are incomplete or inconsistent when boiled down to the level of the individual that RPGs focus on. Messy in the sense that I have to do just as much work to flesh out those missing details for historical materials as I would do for a fictional setting. Since there only so far I can bend a historical setting to setup a situation I am interested in running as a campaign, I find it easier to do that with a fictional setting.
So when one combines picking which medieval culture one is depicting with incompleteness of source material the result is that creating a fictional setting like Harn is just as valid as far as runing a Medieval Authethic campaign goes.