First thing you should know is, I returned it.
There have been a lot of big steps forward for this game, if you ever played Hearts of Iron 3, which had one of the worst user interfaces I've ever seen. Hearts of Iron 4 is a little bit of an upgrade.
What's that you say? What! It looks so pretty!
Well yes, they gave it some nice coloring, a little bit better graphics, and some depth. But let's take a deeper look:
* Some technology trees are horizontal. Some are vertical. Seems to me different people were in charge of each interface.
* Buried menus. Now this is what really sucks about this game. There is a ton of depth, but they make you click through various submenus to get to it. If you are new to the game, you really don't know if you are missing something huge by not ticking a box layers deep.
* Flooded menus. Some of the menus are vertically scrolling and go on for miles. They recently released statistics on the popularity for each played nation. I have to believe that the USA statistics are based on how annoying it is to control all the armies everywhere.
* Countless menus. There are just a shit ton of menus. Holy crap.
Needless to say, I spent most of the time trying to figure out which menu was where, if I was missing anything, and pouring over YouTube videos while turning the game off so it didn't burn my two hours until refund.
I actually spent a few hours on YouTube trying to figure this out, and I was beginning to get the hang of it, but I felt I needed even more training. In short, I just didn't have it in me to go through all these goddamned menus, and I couldn't imagine myself doing it when I have other games to play.
I like paradox games. I like Europa 4, Crusader Kings 2, and Stellaris. The time to learn how to play those games is probably five hours or less (a lot longer to master, but at least you can start playing!). Hearts of Iron 4 continues to have a very high barrier of entry. Buyer beware, if you're not prepared for it. There is a tutorial, but it is woefully short, and they do what we all find lame and lazy: they just link their in-game browser to the wiki.
And paradox, hire someone who knows what a GUI is. Also, how about making a tutorial that's more than five minutes long?