That's what I noticed in my game, too. The Hard AI just left a bunch of militia planets alone that were right next to his capital.
I made a new game now with 4 hard AIs, and after some time they now act aggressive against me. 2 of them started simultanus attacks, including Titan, Capital ships and a large number of small ships. One of those fleets defeated my Carrier fleet, the other one crushed my planetary defenses and I lost a colony. Of course, I rebuilt and retook all
I think the AI plans more now and waits a bit more until it attacks. And it seems to use much math for this kind of attacks. One time they attacked a twin SB system, and destroyed one of my SBs exactly 5 seconds before they lost their last ship ^^
But can´t tell if it is much harder now or not. Will try more.
Interesting. I did notice in my first boring game that the Hard AI was making ships according to the stats, but he seemed to be throwing them only at other AI players.
Something going on with this beta patch and the AI, that's for sure.