I think my main gripe with the game is the achievements, far too many of them rely on RNG, more so than their other games in my opinion.
As an example, I have been trying to get the Destroy the People of Earth! achievement - As the Commonwealth of Man, destroy the United Nations of Earth with your ruler being a human with a symbiotic brainslug, without blowing up their final planet.
Seems quite straightforward, however, to get the brainslug you have to be the first empire in the galaxy to discover the abandoned settlements anomaly, after many failed attempts I eventually resorted to playing no other empires so only CoM and UNE spawned, even then I didn't find it by the time the galaxy had been fully colonized, but by luck after my first war with UNE it seemed one of my science ships inherited the anomaly so I was able to get the brainslug and elect a ruler with it.
After another war I had reduced UNE to around 20 systems, and had claims on them all, so was just waiting for the truce timer to expire, 6 months before this happened UNE imploded and I get a message stating they no longer exist, so couldn't destroy them to get the achievement...aargh!!
As I had a backup save I was able to save scum back to before the second war, but each time the same thing happened, so finally had to keep trading them systems with colonized planets every time they fractured to ensure they didn't disappear again before I had a chance to destroy them, not really in the spirit of getting achievements but I couldn’t take another CoM playthrough.
There are far too many similar to this that rely on very random events to happen to even have a chance of getting the achievement, which you don't know until you have already invested countless hours in to the campaign. [rant over]
Other than that I'm quite enjoying it