That is correct. Ensuring consistent quality and balancing between a huge number of civilizations is hard and even more so with a tiny amount of people to work on it. We have scripts of course, but before I joined the team, there were a lot of missing assets, broken models, unfinished ones etc.
I had been advocating for years when I was a modder for more civilizations integration, kinda gave up on it when I joined the team because some team members were really adamant because of the reasons I quoted above. Then the kushites got in, and finally the han.
Another big barrier to the addition of new civs is the scrutiny they go under. Everything is fine with a mod until it gets in SVN, or wants to get in it. Then you need to redo everything.
Another thing that is much less mentioned and is starting to be a problem is the art "quality" in the performance sense. @Alexandermb did an incredible job in updating the assets to be more beautiful and more historically accurate (at least I think so) but we now have FPS quality assets in an RTS game, which on the lower end computers is a big issue because of the number of Drawcalls (200 for ships due to lack of instancing) We have models that have useless unseen faces, too many textures variations too (Although I absolutely love it) Shields are made of umbo, ring, front face, back face, which all use a different texture, maybe they could be mapped in a texture atlas, but then we'd need a lot of meshes. Ideally we could control the UVMap position in the actor, but I'm not sure how costly that is.
More civs, more assets, more textures => less performance if not careful.