I love The Serpent's Shadow, one of the earlier books in the Elemental Masters series. I own it and re-read it. "Steadfast" feels like a lesser version.
The themes in it -- abuse, class discrimination, etc -- are well handled, but the spark of magic and adventure isn't quite there. And as some people have noted, there's a strong argument for including trigger warnings.
I've been thinking about it and my opinion is three books are struggling to get out. The first is the traveller plot. So much could have been made of this. The book opens with it and then the idea fades. Same with the circus idea -- I don't want to be too spoilerish, but I definitely thought the novel was tracking somewhere else with the idea of the circus master. Shame it didn't go there. What a great potential villain. Finally the story the book seemed to settle with, of our heroine dancing for her life. If that was going to be the story, I'd have rather started with it.
I am a huge fan of Mercedes Lackey's books, like "Sacred Ground", and was a bit disappointed here.