I mean, this was my very first C-drama, bromance one too. I had very little expectations for the series and I loved every second of it. Plus, I am planning to re-watch it so that I can experience the beautiful chemistry of the two leads once more.
Let's be real here: the plot and production value is subpar. It is apparent that the budget was cut halfway through so the CGI is oftentimes horrendeous, there are no extras in certain areas, the props are also cheaply made. Also, this series balances on the very edge of all the issues that are censored in China (BL, magic, ghosts, time travel, etc.). Meaning, the series is very very different from the novel and some things are not really explained to avoid further censorship. One thing I did not understand in the series was how come that Zhao Yunlan is a Guardian.. HOW?! He is just a regular human in the series. Nothing supernatural about him, really.
Some of the dubbing was also quite bad - why were some phrases/voices dubbed?

The best thing about the series are the main actors. They are SO GOOD! The way they portray the BL relationship is amazing and everyone knows that they are not just friends. All the little glances, touches, stares, reactions communicate well that the guys are actually in love with each other.
I completely understand why this drama is so loved but also criticized in terms of the plot development or production. Yet, I think that anyone interested in BL/bromance/censored dramas should check it out.