Ofc, this is prime example of being a dick, but posts like this are already a rarity (i could be wrong as i don't read every section of the forum), but the thing is that you actually never know why any feature turned out bad. What you're supposed to write then? The harshest critique is usually on problems that was kind of expected (AI-related) or repeat again and again (like major bugs).
Repeat criticism of weak parts of the game should be fine, again just don't be rude.
"the ai is poor, it needs to improve". Fine, this is how you feel and you've said it in a way that isn't rude to anyone, no need to elaborate.
"the ai is poor and these are examples of why". Even better, especially if coupled with what you would rather see in those examples. If you find a mechanic not fun, you probably know why after all!
"the ai is poor and pdx suck at making ai". This treads a line, if in doubt don't.
"ai programmer is incompetent and should be replaced". This is toxic and clearly so. I have seen this on more than one occasion in the hoi4 forums.
In short your not expected to come up with ways to fix the game, though constructively worded feedback is always best. Just don't be rude!