In my last few games, I've really payed attention to how the AI behaves and why I've been able to win wars which, on paper, I would have thought I would lose. I've noticed a few things that really stand out and so I thought I'd share them here. As I know the AI will have some changes in the coming expansion, I'm hoping this information can be helpful to the devs.
I've listed them in what I consider to be most important to least.
1) When attacking space ports / planets / mines, the AI ignores fleets - this really puts them at a huge disadvantage. I've been able to defeat much larger fleets than my own simply by waiting at my space port for the enemy fleet to arrive and then taking them out. They will simply ignore my fleet while focusing on whatever their objective is. In these cases (which is basically all of them), if the AI would have simply changed their objective to first take out my fleet and then take out the space port or invade the planet, they would have been much more effective.
2) AI Economy - the main problem is, as many have posted, is the AI saves far too much; 100s of thousands for no reason as there is little use for saved money in this game (save for the far overpriced tech trading and payments for wars / sanctions). As another post had brought up, the higher difficulties aren't even any more difficult because the AI just saves more; it doesn't use it's money to buy ships! If the AI actually used it's money to buy ships, it would be incredibly more effective. Another big issue is the AI puts too high a priority on troops and too low a priority on military ships. If you control space, you protect your planets; troops only go so far. I've seen stacks of 20 - 30 AI troops invading undefended or lightly defended planets... that one invading force has more troops than my entire army combined! Yet, they can only take out undefended planets as my actual military fleets are strong enough to take out theirs and prevent most planets from being taken.
3) AI expansion is slow - this seems to revolve around two main things. For colonization, the AI just doesn't make enough exploration ships. It seems to only make 2; definitely not enough. I have 4, at least. I build more, if I can afford it. Exploration is key in the beginning stages to find all those good spots. For mining, this revolves around the fact that the AI doesn't build enough constructors. In my games, it has about the same amount as myself, even though it's economy is 10x mine. If it were to build about 3x more construction ships, it would expand it's mining operations fast enough. The AI can definitely afford these
4) The AI doesn't have defending fleets, ever - fleets only attack. Star ports only work well when defended by fleets. Otherwise, they can only defend against small attacks; medium to large fleets can easily take out star ports and other static defenses. High priority targets need fleets to defend them as well... and the fleets need to actually stay at the planet, not jump out to attack a stronger fleet in the same system. Fleets together with a space port are a strong defense. Also, small fleets to defend areas of space with important mines would also be useful. As is, a small attacking fleet can easily travel around, taking out all those undefended AI owned mines. So, as a whole, a few well placed defending fleets could help the AI on defense.
5) AI ships have too few shields and too few weapons.
From my observations of the AI behavior as well as checking in the editor, these are the main things I have seen. If these things were to be changed, the AI would be much more challenging than it currently is.
Anyways, I hope that information can be useful for the devs. Maybe some other players have noticed other things as well