The ai is just hillariously bad.

*update
Since 2016 they released some DLCs and some updates, however the AI did not get better. It got WORSE. I really do not understand how one can program such a bad AI. And Paradox has a dedicated AI programmer for HOI4. It really destroys the whole game.

The ai does constant suicidal attacks until their units have no equipment left. When they are on the defensive they constantly shuffle their units around and thus have a really high attrition and are rarely entrenched. Naval AI just is pratically just braindead anyway. If you are able to defeat the english surface fleet with the german surface fleet, then there is just something very wrong going on (and thats coming from an actual German :D).

I have played a lot of HOI4 and the AI is not a challenge at all even at the highest difficulty and while giving the AI a lot of extra bonuses. Especially if you compare this to HOI3 where you actually could have a real challenge at the higher difficulties.

Everything else is good in this game, but the AI just drags that down.