I'll definitely try the demo but my main complaint so far is that, after a couple gameplay videos, I don't understand what the selling point of the game even is. Where is the advantage over Civ? What innovation is there in Millennia that makes it a good addition to the 4X genre? What existing problem has the design team solved better here?
That's in part on the videos, but in part it's the IMO abysmal marketing surrounding this demo. Why is there no feature or design concept being given marketing attention? Why are there instead only very generic descriptions that could fit any historical 4X? Humankind, in contrast, did that very competently - they really rammed their main selling point (combining cultures) home, from the game description, to official screenshots, to first gameplay reveals. I may not have liked the result, but HK told everyone why they should play it. Millennia, so far, doesn't. The Ages system is the closest it comes, but while the idea of Variant/Crisis Ages is fun, it doesn't seem very different from Civ6's dramatic ages. Maybe it's very different, the demo will show it, but the game's marketing certainly doesn't help it.