SOOO many people are confused about the 'change' of actresses.
The first season had Naksu in someone else's body. So she looked like said person. As all the soulshifters do.
It was wonderful, & Jong So Min did a great job. But let's remember that Naksu in actuality looked like Go Yoon Jung, okay?
So, that ends with her petrifying like other soul shifters.
But Master Lee, plus the powers of the owner of the original body, makes it possible to bring back Naksu/Bu Yeon, albeit without any memory of the past or her identity. The soul that inhabits the body being Naksu, it shapes the body to look like Naksu (very necessary for the plot). THIS undoubtedly was planned from the beginning, because otherwise everybody would have immediately recognized her as Mu Deok! I did find this a bit inconsistent, as the Queen doesn't manage to get her original form back, but I suppose it could be explained by the body of Bu Yeon being turned to stone & having to reconstruct itself.
So all the nonsense about the actress 'changing' is just that - coming from people who do not understand the underlying plot development very well, & possibly failed to realize that the actress in Part 2 is the SAME one playing Naksu (as Naksu) in Part 1!
As for the way Naksu as Bu Yeon acted, good grief! She totally behaved as Mu Deoki had done (or as Naksu as Mu Deoki had done). She didn't have the anger and drive of Naksu because she lacked the MEMORIES of Naksu, but she didn't act like the real Bu Yeon either (as much as one can tell, seeing we only saw her (as Mu Deoki) briefly. I totally enjoyed those episodes so much, because she was reminding Jang Uk of what drew him to Mu Deoki without him realizing what was happening.
The final 2 episodes had some WTF moments, but they weren't overwhelming & mainly had to do with the somewhat illogical plot around the House of Jin & that Firebird/Dragon. It did seem that Jang Uk managed to destroy the mages much too easily, as well, which could have been improved with another episode - but since he'd been touted as all-powerful through-out Episode 2, that didn't bother me overmuch.
I loved the way all the secondary plots & characters had their own stories wrapped up. So many of these fantasies end sadly or ambiguously - or 'they were reincarnated together' ~sigh~ This one didn't do that, and I'm grateful.
So overall this is one of the best high fantasy dramas I've seen - including Game of Thrones, The Witcher, The Rings of Power, as well as several Chinese ones, let alone Korean. It wasn't dragged out too long (GOT, & every Chinese fantasy I've seen other than Ghost Bride), didn't destroy itself in the end (The Witcher), and didn't have a book that it mangled (The Rings of Power, Wheel of Time, Game of Thrones once Martin left). The Hong sisters know their craft very well indeed. Kudos to them!