It's been years since I played it (I also got it at launch, and I wrote it off shortly thereafter). But what I remember most was an appalling interface. It seemed like there was eight million battles, and no way to have any overall view of what was happening. I couldn't track battles unless I zoomed to each one individually, and WW2 is far too big for that to be practical. An improved outliner might have saved it for me, but if I can't track the battles, I can't play a wargame.
TBH, I feel like I should maybe go find some HOI4 Youtubers and see what it looks like when they play. Maybe it's been improved, or maybe I just didn't understand how to make the UI work. But I did give it a couple tries, and it just felt hatefully bad to me.
Edit: Pulled up some Youtube. I literally do not know how they are playing this game - it looks more like obsessive actions-per-minute Starcraft than a proper strategy game to me. Battles don't exist unless you happen to be looking at that part of the front, and you can't have any sense of more than one battle at a time - it's just spastic clicking.
Edit 2: Found a HOI3 screenshot that shows what I'm thinking of. That box in the top right was missing from HOI4, and it was literally unplayable for me as a result. (And no, that's not a meme - I actually mean it.)
https://i946.photobucket.com/albums/ad308/CptEasy/Blitz Carnage/13.jpg