I really liked the first season of Arthdal Chronicles, found it entertaining, the lore was interesting and it kept a very decent pace considering the length of its episodes.
I had seen the change of actors and I was slightly skeptical just simply because I was not sure if the characters would be able to keep the consistency they had being different actors. This was partially true.
I like both Lee Joon Gi and Shin Sae Kyeong but after watching the season I'm still left like I'm not quite sure how I feel about them in the roles. For the most part I think Lee Joon Gi did a good job as Eunseom but I was not feeling it with Saya. This could be totally the fault of the writing though so I do not blame him for it entirely. Samely, I felt Shin Sae Kyeong did a good job for the most part but the character felt very unemotional and superficial compared to the characterization I remember from the first season, again, it might not be her fault, but it's how I felt it.

For the most part I think the worst downgrade in this season was the writing and plot in general. There were a few problems I found in the cinematography as well (like the weird blur at times) but I can give those a pass. The plot though, it was absolutely dragged to fill episodes to get to the minimum. Back and forth constantly to then a rushed ending that could have been gradually done in a couple or three episodes instead of condensed in one. There were very weird plotholes and bad execution. For example, there is that time when Tagon wounds Yangcha and Tanya has a mini conversation with him and I guess Tagon was waiting in the background looking at the watch and waiting for them to have their moment or something... That was not the only instance of something similar happening and it was very sloppy.
The plot was not as solid as the first season, I guess they couldn't keep Tagon in the grey area anymore because otherwise it wouldn't make of him the pure classic evil to be dethroned, but I thought it was cliché and overall generally boring. His relationship with Taealha was weird as well, one second they were going to kill each other and then they're ok the next, there was not enough insight into it to understand fully how they could be normal when two seconds prior she was going to stab him or something.
I think they had enough time to do a decent prologue instead of an open ended hint to a possible next season that who knows if it will happen. Also what happened with the prophecy that said about the three of them together? I was kind of confused really because the ending suggests something very different, and I was disappointed that they were taking Saya in that direction, repetitive? boring? cliché? I don't know, I liked the first season because it was a bit fresh but in this one they were falling into very repetitive stereotypes. Saya was a very interesting character but in this one I couldn't care less about him.
Then you have characters that appear suddenly at some point and it's like major reveal but you're like "what's the point of you being here now", like most of the momo tribe for example, ones save the day and die, the others appear only in the last episode.
I kind of wish also they had gone in a more mysticism way at some point, not so much what they did with the gods and Aramun and all that that got repetitive real quick, but I would have liked to see a lot more of the neanthals living in nature for example. I remember more of that and a lot more lore in the first season as well.
Apart from the couple of neanthals that I always liked to see, in this one I generally liked Yangcha and Nunbyeol as characters the most.
The acting was generally good, there were moments that were cool, characters that I liked, production was generally decent enough and I'm ultimately happy I could see this story to more or less an end.
I'll rewatch the first season at some point but I probably won't bother with this one.

It's watchable and if you have watched the first one and want to know how it continues then this is for you. If you haven't watched the first season I would avoid this one since it's a downgrade, writing is worse, it's slow and not much really happens when you strip it from their back and forth. Go watch the first one instead.