I started this drama because I have heard that never ever have the same Chinese actor couple got the chance to stare in a total of 4 dramas! Of those four, this "The Eternal Love" series has 3 seasons; the 3rd one just got confirmed. I really wanted to see for myself, which couple is this lucky!
Here is my verdict: I would also recommend not watching it if you into more "intelligent" romance dramas if you know what I mean. You have to completely throw away your logical mind to enjoy it. Unfortunately, I was given this tip before watching it. I have to say that I was disappointed. I would, therefore, rate it as an average of 3/5 ★★★☆☆.

Like my last review, I will use my own seven criteria to provide some details to my score. Please refer to the footnote for more information.

1. Fantasization 2/5 ★★☆☆☆: I really like the start of the drama. I think the story was unique to combine time travel with two souls in one body setup. It started out really interesting in terms of her interactions with the ML Muo Lian Chen. However, I was disappointed with how this was not "well used" later on in the drama. A few episodes into the story, the modern Xiao Tan was dominating the scene, and historical Tan Er just more or less leave her body to Xiao Tan. Is this your natural reaction if your body gets taken by someone else? Even if Tan Er has a weak personality and wants to die, Xiao Tan uses Tan Er's body to sleep with a man she does not even love... I became not very convinced by how the story was going on from then onwards.

2. Emotional Rollercoaster 4/5 ★★★★☆: I like the main couple and found their chemistry between them really sweet. This has helped me to finish this drama and ignore a lot of inconsistency and plot holes. At first, I found Xing Zhao Lin's (starring the ML) acting very stiff and not very convincing but was shocked to know he was only in his teens when filming this drama! The fact that he is not that good looking (at least I find...) but still managed to make this drama such a success is a proof for his talent. I noticed he has a lot of potentials (which was proven to be right after watching the 2nd season).
As for the Liang Jie (starring the FL), I am not sure why, but I was slightly annoyed by her character. This is probably a bias arriving from my negative experience with her in Handsome Siblings. This time, I have a much better impression of her, especially her exceptional acting skills playing both Tan Er and Xiao Tan. Even though I still find her acting a bit over occasionally, I enjoyed the sweet and dorky romance scenes between her and the ML. Many cute scenes gave me fluttering, and the kiss scenes are something to look forward to.

3. Sleepless night 2/5 ★★☆☆☆: Despite the makers trying to make the drama more dramatic (e.g., the demon king, the mysterious soul stone...), I could not get into the seriousness of the story. This is primarily due to the plot holes and over-the-top story development. Another reason is that the entire drama just felt really cheap. I don't have a problem with low-budget dramas; in fact, many of my favorite dramas did not have a high-budget. But this one was way too obvious. The sets, the costumes, worst of all, the few extras! Can't they at least hire a few more extras to make the cast looking like they are actually kings and princes? For these reasons, I just did not feel like binge-watch the entire series.

4. Natural and intelligent storytelling 1/5 ★☆☆☆☆: The story has a lot of flaws. As if time traveling is not already complicated enough, the main characters are from another world? The MLs are the reincarnation of these people? What's up with this flute? Utterly confusion.
I also don't understand why Xiao Tan's family is this hostile to her, half-sister and stepmom aside, even her own father? A lot of the scenes are cut abruptly; thus they feel disconnected from each other.

5. Loveable supporting characters 3/5 ★★★☆☆: this one is a bit difficult. Jing Xin, the maid to FL is one of the most loveable supporting characters I have ever seen. There are times, at which I love her more than the FL herself. Yin Ning did a marvelous job, and she definitely deserves to play FL in a drama. Then there is the magical elderly Liu Qian Shui, who is the comedian in the show. The entire viewership is making fun of his hilarious accent. However, I am a bit disappointed with the remaining supporting roles. A few of the essential characters are even bad at acting, e.g., the 14th prince, FL's father, or the king.

6. Proper pacing 4/5 ★★★★☆: given the length of 24 episodes, the pace was appropriate. I recommend future C-dramas to be around 24-30 episodes. The ending felt a bit too quick esp. because the story was all over the place.

7. Solid finale 1/5 ★☆☆☆☆: the ending of this drama was a disaster, if I may say. I am not going to spoil this but just be prepared for a fast-pacing, jumpy and illogical development of events happening one after another. On top of that, the ending of some of the key characters, despite predictable, was unsatisfactory. This was coupled with the cheap-looking CGI that was heavily used at the end.

One additional point that I usually don't talk about but is worth mentioning is the excellent OST.

In general, I would say it's not a bad drama, if you are into a simple, sweet, playful love story with a refreshing setup. But if you are into some intelligent storytelling or good acting, I would suggest you skip it.

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Kotori's drama review seven criteria definition:

After having watched many dramas and also written several reviews. I have detected that I rate dramas based on specific patterns. I formulated those into seven criteria.
1. "Fantasization": the degree of how much I care about the fate of the main leads, how often I have to think about them in my everyday life, and how much I wish the characters are part of my real life.
2. "Emotional Rollercoaster": the degree of how much my heart was moving from the acting, scenes, and story development (e.g., in pain, pounding, fluster etc.).
3. "Sleepless Nights.": how much I have the urge to binge-watch this drama.
4. "Natural and intelligent Storytelling.": how logically the story was able to proceed and the roles are acting within their characters. I don't have a thousand question marks when watching the drama. The feeling that the makers take us viewers seriously and that we do not always think that this looks fake.
5. "Loveable supporting Roles.": how much I love and care about supporting characters. They are not merely tools to help the main characters but are 3-dimensional and have proper motivations behind their actions.
6. "Proper Pacing.": how well the drama is pacing and not watered down to fill the number of episodes.
7. "Solid Finale": how properly the drama builds dramatic and finishes with a satisfactory ending. I have just watched too many dramas that starts to lose its grip around half or 2/3rd of its way.
Obviously, many other criteria could be important as well, such as unique story setup, visual elements, camera work, music, costumes, etc. Some of the points are indirectly included in the criteria, and others are not this important for me.