I call it the curse of Action+Yuri series. :-( Almost all manga I've read that combined those genres got axed before their time...
It feels like there's little overlap between the audiences of the two genres, so a series that tries both ends up unpopular in either...
Don't get me wrong, I do love this series a lot, but I also think it had that ongoing problem that there was no big plot going on in the action part. In romance, we had this 'I will make her fall in love with me' which is a cute trope, but it was not the main focus. And at the action part, there was never a big threat or plot coming together like I was all along waiting for some big enemy to appear, and I thought that this whole monster rising was because some evil man was trying to attack the country and start a war, or ruin the Black Knight, etc. And I know it's basic, but I was so ready for the enemy to realize Clair is important to Frost, so they get her and other typical stuff that would have made this more interesting. Because even though I love this manga a lot, I always missed the big threat (or if they actually had one coming later, maybe they should have introduced it in volume 2). I think this story needed a bigger threat than the dragon, and maybe more drama? Who knows. I just understand why people got bored with where the story went. And even if it hadn't gone axed, I can't really see where the story could have led, which is maybe a problem. But I know it was the writer's first manga, and I'm glad they at least can have an ending to the story.
Action yuri tends to have a lot of the same problems that I notice in action isekai series. I think a lot of it comes down to the fact that action is hard to do well in a manga. In animation or live action you can just have a well choreographed fight scene where the spectacle of the fight carries the scene, but if you draw that same choreography as a series of unmoving manga panels, it loses most of its energy and looks way worse. You need extra elements to keep it interesting most of the time. Maybe if you're a once in a generation talent you can draw something like Berserk where most of the action takes the form of a short series of incredibly detailed and violent impact panels and let the art quality carry it, but for most artists that isn't a real option.
Most of the yuri action (and bad isekai) I've read loves to throw the protagonist against mindless animal monsters that have no personality or complex thought. Which is incredibly boring. Human enemies with big personalities are way more interesting. With human enemies you can have banter, you can have a clash of ideologies or personalities, you can have more personal stakes, you can resolve the fight in a wider variety of ways, and you can have the enemy use intelligent or interesting tactics.
Another thing you can do (that this manga doesn't) is to do Jojo's Bizarre Adventure style puzzle fights. The protagonist has a weird superpower, and they have to figure out how the enemy's weird superpower works in order to find a clever solution. This adds an additional layer of mystery and tension to the fight and you can write a whole manga around this kind of thing (see Jojo's, HunterxHunter, Kaiji, honestly half of all the shounen out there).
I wish action yuri was more of a thing, but I understand why it isn't. It requires an author who understand both how to write a compelling romance and how to write compelling action, and having even one of those skills is rare even among published manga artists.
I don't even think you need all of this to make an entertaining Yuri action manga. Though I also don't need every series to be a 10/10. I'm perfectly happy with a decent action series with Yuri romance in it. I'm someone that loves a lot of action series and romance series that are varying in quality, I'm curious to see what people make and am often pretty open to seeing a varied range of quality. I'm probably also more lenient than some.
Also I think action through manga art is fine, one key element to fun action is choreography. Sakamoto Days isn't my favorite manga and I'm not sure how good the anime adaption is, but I know that the manga has some really fun fight choreography. How the characters interact with the environment around them throughout the fight is also a fun element. Some of the fights in that manga give me some vibes of old school Jackie Chan.
