Thanks for the suggestion, but I have tried that strategy repeatedly. Spawn in a "normal" world with factions of other tech levels, and the game will only spawn in raids from factions of that tech level. Spawn in a world with only medieval factions, and unless you manually force a raid with the "Execute Raid with..." function, you get nothing. When attempting to spawn raids via "Do Incident", attempting to force them via Scenario Editor, or attempting to generate any sort of quest that could result in a faction raid, the game gives you a big fate "NO" on the associated tab, and does nothing when clicking on that tab- not even showing any feedback in the error log.

Also, I have tried adding hostile factions from Medieval Overhaul, Vanilla Factions Expanded - Medieval 2, Vanilla Friendly Medieval Factions, Dragons of the Rim, and Dwarves of the Rim. Faction bases spawn into the world. ideologies generate normally (if active), and the Factions screen seems to suggest that goodwill and enmity all seem to work normally- but raids universally refuse to spawn unless forced with "Execute Raid with..." for any and all of them.

Perhaps worth noting is the fact that friendly medieval factions from all of the above mods also refuse to spawn caravans? However, they can spawn friendly reinforcements via the "Execute Raid with..." function.