Let me start by saying that I really enjoyed this series, and have binged it all the way through a few times now. First of all, it's just such a cool world! The different gods having representations on earth to work through, gaining or losing power based on the number of followers; the concept of having this Twelve Holy Knights brotherhood, where they each have to have these distinct personalities facing the public but are different underneath; even just the idea of those knights passing on their (weird) duties to the next generation and then going out in the world to retire, is all just so interesting and fun.
It's not a deep read, and while there's a few big twists that involve things that seem unimportant later on coming back around which were really exciting and I didn't see coming, most of the time you can't take anything too seriously. (It actually feels very Fast and Furious in a lot of ways, where someone can do an absolutely terrible thing and be an enemy, but then somehow be friendly with everyone by the next novel.)
The last novel is definitely my least favorite, though - I'm not sure what happened, because clearly the author wanted to continue writing in the universe based on immediately going into the sequel series and the spin-offs, but it really felt like she didn't know where she was going in the end. There's a lot of things that get brought up only to ultimately go nowhere, there are some high-stakes and consequences that just end up dismissed, and the final resolution for the problem they were facing...doesn't really seem like it would work long-term.
Also, between this and 1/2 Prince, I really don't think the author should be allowed to write romance, because (in my opinion) it's always rushed and doesn't make sense. It's surprising and disappointing, because she can write really great relationships - the bond between the Twelve Holy Knights is half of the reason I enjoy this story. Frankly, you can definitely argue that the relationship between Grisia and Lesus is queerbait-y, since they clearly care about each other so much and grow even closer over the course of the series.
...But then we have Leaf and Princess Ann who get like...maybe two lines back and forth and show no compatibility, and are somehow together by the end of the novel? (But not really, because the ending means that Leaf has to stay in Leafbud City for the rest of his life, so how does that work with her being the literal princess of another country lol.) And Grisia and Charlotte, she kidnapped him, tried to kill his friends, and destroyed his city - when we later get a flashback of a nice scene with the two of them, it's useless development because he literally already killed her. And with Charlotte #2, from the beginning the only reason Grisia is even around her is because he's pursuing another man who happens to be in her party - he definitely spends way more time thinking about that guy then he does Charlotte #2. She doesn't even get a mention in the epilogue, but somehow they're married in the sequel series?? Yu Wo, if you want people to care about your romance pairings, you have to actually develop them, not just say that they're a thing. It annoyed me so much that I couldn't keep reading the sequel after that, I just skimmed to get to the ending for completion's sake.
If it weren't for the ending (and the sequel series, which tries to answer some of the threads left dangling but mostly just ends up not really working for me), this would probably be an 8 or 9/10 series. With it, though, I have to drop it to 7/10. Still worthwhile, and I'll still re-read it again! ...But it's hard to get that ending's taste out of my mouth, which is just unfortunate.