I am pretty nervous about EU5. Lots of good stuff, I am just concerned about the AI. If the AI isn't good, then the entire game will fail. That was always the biggest weakness of EU4 (even M&T) was once you got a good snowball, the AI could never figure out how to do anything. Especially with their insistence on not having any railroads at all, if the AI isn't actually good at using the systems they implement, then the player will always win no matter what they do.
Also I am quite concerned that they haven't shown any gameplay or dev diaries for the building-based countries or the army-based countries. Playing as a bank or something seems like a really cool idea, and a great way to bring some new gameplay to the genre, but they might be having a lot of issues with it since there have been no updates on them that I have seen, beyond the initial dev diary describing that they exist.