I have big problems with current state of the game. It's just too easy, somehow all that changes (and I like em) give you even more control about everything and you can crush pretty much anybody, because AI sucks (and rightfully so). ...
It's hard to say... It seems to suffer from the same problem that EU3 did, in that once you're already big, it's harder to fail. So if you start as a nation that's already big, and get bigger, then it's just going to be even easier. If you're lucky, maybe another nation will blob hardcore and be at least a passable threat.
But if you pick a small nation tucked in an extremely politically hot area, it's going to be much, much tougher. Perhaps even more than EU3 was, because of all the new diplomacy mechanics which can both work for you, and work against you.
I find my Urbino game to be pretty intense. I haven't died yet, but alliances shift like the wind, and I've only just barely managed to squeak out of getting my ass handed to me a couple times (admittedly to what feels like cheaty peace deals). Rebel stacks are frightening when you're small, and I've been nearly broken once, and successfully broken another time. It's a diplomatic challenge trying to find passable allies that are actually close, trying to keep major powers happy, and finding the best possible expansion route that doesn't piss off a nation who has four times my forcelimits and can squash me.
And the larger I get, it feels like there's no end to the threats. First it was just a couple neighbors in Italy. Then it was larger Italian powers, who despite staying the same size, managed to match my forcelimits, or even exceed them. Now I'm starting to draw the interest of the minor powers. I fear soon, the Holy Roman Emperor will find a reason to dislike me. Or Austria. And I
know eventually I'm going to butt heads with France. It's inevitable. That's going to be a fun one indeed. I just hope I'm large enough, or have some competent allies.
I suppose it isn't so much any difficulty as feeling like I could get demolished at literally any moment, even if it may not actually ever come.
The latest patch does seem to have made the AI more competent at its job, so there's that. I suspect there's always more room for improvement.
As for Muscovy, I think part of the problem is the later start date, if you're comparing it to EU3. In the later start date, the hordes are fragmented and have no technological edge. I believe historically, the hordes were already on the decline by this time.