Great Game - Spent a lot of my time playing this game but i'm not sure how other people have like 1000-2000 hours on this game as it feels like I've played this game for over 200 hours but I've only actually played just over 50 (as of review).

Recommended mods:
The Great War Mod
Road to '56 Mod

Sound - 6/10
Music can get a bit repetitive as i keep hearing the same music track over and over but i'm sure there are more soundtracks in the DLC. I'm also sure that there are some music mods out there but i have not been bothered to look.

UI - 9/10
The UI can be a bit overwhelming and complicated when you start the game but have probably one of the most simple Paradox Grand strategy games' UI in my opinion behind CK3 (Crusader Kings 3) which i have played on Xbox gamepass for PC.

Gameplay - 8/10
The gameplay is really fun and strategic after you get to 1939 (The historical start of WW2, but you can play against history)
The first few years in game can be a bit boring when playing historically but there is a 1939 start date to get right into the action. I do find problems with not being able to break through an enemies lines even though i have double or even triple their division, but i am sure that there is something that i am not doing correctly and that its not the games fault.

Performance - 5/10
even at low settings i get quite a lot of lag and most the game is played at 20-30 fps and i have a pretty good PC. It can get even worse late game going down to around 5 fps sometimes.
Edit: I have recently upgraded my PC and now the game runs great. But the game still needs a very powerful PC compared to what the game actually is (Horizon Zero Dawn runs at a higher fps than HOI4)

Overall, I recommend this with the Road to '56 mod if you want to play WW2 as it adds a lot more country focus trees whilst also allowing you to play to 1956 rather than 1949 end date (ends on 1st of January 1949).

Hearts of Iron 4 came out in 2016, and its mechanics allowed for a far more on the nose RPG experience (all Paradox games are RPGs, just to be clear than its predecessors. HOI2, which I admit I have extensively played, was very poor in the amount of "routes" your nation could take through prewritten events - there was some exceptions, like having a decision to escalate the Khalkin Gol incident as Japan, but you could just declare war on your own volition anyways so it doesn't matter. In vanilla, unless you wanted to go through a boring and unfun process of clicking a slider once every two years, there was no way you could change a nation's ideological role, and once you did it, you had nothing to show for it but changing the minister portraits and, if you became an authoritarian country, the power to declare total war at will.
Of course, Kaiserreich had its origins on Hol2, but I think the original mod was more intended as a way to get conflicts in areas of the world that normally no player would pay much attention to, and give something to do to countries that, in the base game, were there either to be spectators or to be devoured by Germany or the Soviet Union between 1938 and
1942. You get Huey "wholesome" Long to win in the American Civil War, you at best click a few buttons on some events passing quite quickly through the screen, decide to give power to the nazis or don't, and you are let go to either give the game for finished or to immediately declare war to anybody else. There was no intention to build an "universe" out of the socialdemocrat world of Kaiserreich. I think this is something that only came with HOl4's mechanics: the focus tree, where you have to click in short novellas every few weeks to make your nation advance on time, being able to automate war so you don't have to pay attention to the battlefront and you can focus on reading those short novellas, the gimmicky minigames,.. It's something prepared to lead into polandball roleplaying.
Also important to mention is that HOl2 mostly avoids the pure number crunching. Even visually, it's mostly based on sliders, which can be quite imprecise, and bars. There's no way to win by purely crunching numbers and optimizing division performance, it's closer to a board game, you have to think in and about the terrain. I don't recommend playing it because, honestly, Paradox games is electric sativa and a colossal waste of time.