Yes. And somehow they are never capable to explain in details what it improved significantly and how.
So, starter for ten, we can create our own heresies instead of being locked into only the exact heresies that existed. (Yes, people do memey nonsense with this, but any system you can't do memey nonsense with probably isn't flexible enough to do quite a few perfectly reasonable, well-grounded things.)
With RC, we can also hybridize and diverge our cultures rather than being constrained to only those hybrid or divergent cultures that actually existed (which might well be nonsense if things unfold differently).
Combat itself is much less aggravating to engage with (I don't find CK2's "fronts" and "tactics" systems particularly interesting and the latter, in particular, has a bunch of bizarre and counterintuitive effects that can lead to e.g. your army performing drastically worse when you cross a certain percentage threshold for a certain troop type); most of the problems with warfare are in other aspects of the design.
CK3's tech system feels more interesting than CK2's (in particular, the fact that it actually interacts with the culture system).
Hooks and Secrets are a really neat system (with, as usual for really neat systems in Paradox games, some assorted jankiness).
CK3 doesn't have the literal Epic +2 Axe, and doesn't let you grow back your severed manparts by sacrificing people to Satan.
Oh, and the new tooltip UI is fantastic.