I started with medieval: total war when the modding community was already mature and so I could make simple mods in matter of days as there was both an extensive documentation and refined editors, all made by the modders.
Rome: Total War had even easier basic modding thanks to extremely easy to edit txt files (which basically took out the need to use an editor), although the feature set is much larger, so advanced modding can take very long to master.
There I joined my first modding team (XGM) within a few weeks where I was doing mostly balance and some basic scripting.
Advanced scripting can take months or years to master though, as the initial game documentation while effective was not without many obscure points that in some cases are still being cleared.
Modding today is indeed easier than doing it in 2004 as now a good 95% of what the game engine can do has been documented in various tutorial by the community.
All in all let's say that the game developers gave the community a good start as the initial documentation covered a good 50/60% of the game (basically you could make use of almost all that had been implemented in the shelf game) and later on individual modders integrated that documentation with their own research which has been mostly shared.
To sum it up: you had a very easy time in making basic mods but the guessing game started if you wanted to twist the scripts to do things in ways not covered by the shelf game.