The plot is very complex as you have many side stories which are all important and connected. But for me it wasn't too hard to follow, it was just that in some storylines I was more interested than in other... Imo you can also watch this series when you haven't seen any of the other Lost Tomb series (like me). The plot is overall thrilling but some things just shouldn't be questioned. And there are also some cliffhangers, so don't expect a clear fine ending...
The series has everything, really! Mystery, Friendship, Bromance, Comedy, Action, Adventure, Slight Romance, ...

I began to watch this just for Wang Haoxuan (with whom I liked every few scenes he were in ^^) but I ended loving so many of the cast. And also other characters were so cool, I really loved their side stories :D (though most of them don't get enough screen time o.o) The main character though, Li Cu, was just pretty annoying and stupid. Especially in the end parts his scenes were so boring and dumb, unfortunately most of it is important for the plot.
Sooo, the series was very impressive and maybe you should rewatch it to get some points you maybe missed in the beginning, but for me the rewatch value isn't that high because it's such a huge amount of episodes.