Quite a wonderful mod! Since I started using it, it has been a feature set I have enabled for almost every play-through of Starsector I have done. Rather fond of the three factions of the Legio, the Great Houses and Rosenritter.
Given the temporal technology of the Great Houses vessels, it is kind of fitting that the Great Houses are not a playable faction; rather a relic from the past that may still have a bearing on the future. This gives players a lot of freedom (in my opinion) to craft their own stories, and if they so choose to try and restore Kassadar / the Great Houses to their former glory in their own campaigns. That said, I would love to see more Kassadari additions in the future should you be so inclined.
I quite like the Legio Infernalis. In general the Legio is a good addition to Starsector in my opinion, as having a more organized / militant pirate-type fills a niche (dominating aggressive expansionists) that was kind of missing. Thus far, I have tried out the Legio both with and without the Siege mechanic enabled to see what kind of a difference it makes for campaigns using Nexerelin, and have even tried playing as a Legio commission with and without it enabled.
Thus far, I have enjoyed the Legio presence in my games more when the Siege mechanic is enabled. In a Nexerelin campaign, having the Legio Siege enabled is a much better way to allow them to put pressure on the other factions. One setting combination I find particularly interesting, is to have Legio Siege enabled alongside Nexerelin's brawl mode (nex_brawlMode). Makes for quite a bloody conflict in the core worlds!
Also hoping to see some more Rosenritter content in the future! I did notice the Rosenritter skins you created for Diable Avionics, very nice.