If anything, the UI is much much simpler than CK2's (addmitedly I tried CK2 for an hour before giving up in frustration/resignation (partly because the game ran like ass on my old laptop)). Most of the stuff is straightforward like research, picking national focuses and cabinet personell. Construction is silly simple too, production is a bit more complex since you gotta juggle resources, factories, equipment models and the like and all of that comes well before designing and deploying divisions, which is simple once you learn it, but the sheer wall of info you're presented at first can be a bit daunting. Air and Naval stuff is kinda obtuse at first and not very well designed but they're promising a rework of some of it atleast in the next big patch. Combat itself is simply shuffling stuff around, but the tricky bit is figuring out what to shuffle, when and why, and again, a bunch of numbers and stats comes into it if you want to analyze and optimise but it can be simple enough to figure out if you're winning or not at a glance.
So yeah, game is relatively simple to pick up, arguably the simplest of all PD titles atm, maybe competing with Stellaris for that title, but it doesn't lack depth at all if you're looking for it.