The new Esthil update is probably the most fun I've ever had playing EU4. Mechanically, I love how the new magic system is integrated into the mission tree, and the anti-necromancy revolt happens right at the time where most campaigns get stale. It perfectly captures the feeling of being the ultimate fantasy villain while the whole world tries to bring you down.
I've been playing a game as Byzantium for a couple days, and I found it to be remarkably...easy (despite playing on hard difficulty). I no-CB'd the Ottomans immediately and reconquered my cores with little effort. After that, it was trivial to expand while removing the consolidated corruption privileges and surviving the succession crisis.
Now, I've reached the 1400s, and despite rivaling the Mamluks, Hungary, France, and Naples (all of which are fairly strong, at least for the AI), I'm being hit with a ticking complacency score for not having threatening enough rivals. I have to funnel over a hundred ducats/month into diplo spending while keeping my stability above 0 at all times to avoid a nasty disaster.
The fundamental problem (besides BYZ having way too easy of a start, seriously please buff the Ottomans and nerf Byzantium's levies) is that the AI cannot snowball like a human player, which leads to a human player generally having an overwhelming position after ~100 years. My only threatening rival is the Mamluks because my economic snowball has drastically outscaled Hungary and Naples despite a much worse start, so now the game is hitting me with complacency because the AI can't remotely keep up.
I imagine that complacency was introduced as an anti-snowball mechanic, but Paradox put the cart before the horse. EU5 needs anti-snowball mechanics which either slow down early game player expansion and player economic growth, or give AI countries a way to catch up. Instead, Paradox has implemented a mechanic that actively punishes the player for being even remotely competent compared to the AI. It's a classic case of anti-fun game design, and because of it I no longer want to keep playing my current save.