They've really shifted in a more game-y direction over the years. I started with Hearts of Iron 2, where the systems were less abstract and mapped closer to real life. I felt like I was actually running a country and its military. There were more ways to cheese the older games, like the time I played as Guatemala and launched a successful coup to install a Guatemalan puppet government in the USA, but you could avoid those strategies if you wanted a more serious WWII wargame.
Since EU4, or maybe CK2, they've abstracted a lot of the gameplay into those progress bars and point systems, the foremost example being the focus tree in Hearts of Iron 4. I still enjoy most of their newer games, but they constantly remind you that they are games. Gameplay feels less emergent and it feels like I get less agency as a player.