Honestly, I don't think the AI is as bad as it once was (yes, looking at you, USSR. I know you want to move all your divisions to Manchuria. Thx for not doing that anymore).
Naval Invasions + Coast defense is pretty good, for AI.
The only real 3 issues I see, and I hope the Devs will at least improve some of it this DLC (I dunno if you read this podcat or can answer * wink wink *) are the following:
Tanks. The AI should create dedicated tank armies, in order to at least in theory be able to concentrate assaults.
Supply. Currently, the AI has no clue on where to deploy (especially when not fighting on core territory like greece, UK and US etc.). They sometimes yeet around 30-50 divisions into norway or pacific islands. They should realize that it wont work. Nobody benefits of 5 more divisions in this theatre, so fuck off Belgium. Or maybe even give the player the option to say the AI where they should deploy (like in EU4). Example: do NOT stack up on me on Borneo, but DO stack up with my naval invasion army in Normandy.
Commitment. Sometimes they have a really good push or invasion, but they stop bc the numbers don't add up (seemingly?!). But for a player it's quite clear that they could conquer all of france would they only stay and reinforce. The AI should do less offensives and invasions, but try to go "all in", to minimize pointless casualties and replace them with casualties that matter.
I don't really want to personally attack anyone with this, AI is ridiculously difficult to program, but that's what I personally find annoying.