I've only had the chance to play some of the Imperial campaign, and for the first few sectors, I felt it all was a little too easy, I was soon making like 1500 spacebucks a turn and my fleets were merrily taking system by system with little to no opposition.
But everything changed when enemies started to seriously attack multiple systems at once. And not only that, sometimes with large fleets, some larger than my own fleets... Suddenly I was defending various systems, splitting up my forces, making use of turrets and space stations a lot more heavily to give me a boost and edge in fights.
I'm now in a situation where I have little time to think about attacking the enemy apart from small pushes of my territory, while I'm moving fleets around to defend various systems, almost losing some of them to very heavy and hard battles.
It's been great, and I'm thoroughly engaged and would say, that if this sort of pattern is true for the Necron and Tyranid campaign as well, I'm gonna have to sink in a lot of hours and enjoy all of them!
I would say, that as a small criticism, at first it's not really apparent what "Threat" really does, later on, when the attacks started to get more and more frequent and heavy as my threat level goes up, I realize that it basically means more incursions, stronger and more movable enemies, but I would love to have a way to really see how much of a deal my threat level is to the situation. Like see what kind of percentage change it makes, what kind of risk I've got for letting the threat build up to level 1 or 2.
Still, this is fun, and I like having fun!