Ran a couple of games in the last few weeks on Hard difficulty (generally only playing normal) to get more challange and hoped the balanc is fine
My findings:
1. AI was very competitive and puts up a good fight at 50, 100, 200 and ven up till 300 Turns in
2. AI could roll over other AI and consume their territory which is important for forming large enemy empires later - very good
3. Tech progression of AI was believable as there was no traces of sudden technology surge like around release date
4. AI fleet composition was excellent - there was every kind of ship in most of their fleets reom small to doomstack they had them all: escort cruisers, military transports, missile cruisers battleships and a few light cruisers or destroyers
5. Economically the AI empires are blatantly punching above their weight: saw mutliple times empires fielding 6 battleships plus a sizable fleet of escort ships with only a handful of planets with only 1-2 markets altogether. This was especially obvious when I annexed my ally and inherited -200 coins by taking over its planets and ships (some income is of course lost due to the loss of mutual trade routes but that was negligable amount)
6. Planetary invasion calculations were a bit wild in most cases but I cant say I saw a clear pattern in fudging the numbers against the player on this difficulty level so its fine
Altogether, with the exception of obvious ignorance to ship maintenance fees, the experience was well balanced - I would just recommend to allign AI empires a bit more to what theys should be reasonably be able to maintain (somewhere around 20% less then what they do right now)