The Owl House is, frankly, one of my favorite shows. The worldbuilding is very well done, the characters feel genuine, and while the tone fluctuates a little, it's still very good.
The world is, for the most part, consistent. There aren't any obvious later additions that don't fit with the rest of the world. However, there are some aspects of the world that don't feel fully explored. Some side plots are given an episode of exposition but are never completed. Since this is probably due to the shortening of the show, I'm not too frustrated with it.
The characters are another point where the show excels. They're very consistent and the development is good, even for the side characters. While Luz only subtly changes in the first season, the second and third season are very interesting. Some of the development feels rushed (especially in the shortened third season), but the majority of it is well-paced.
Luz is the main character. For the first few episodes, she sees the Boiling Isles as a place to act out her fantasies. Over the rest of the show her character develops, regresses, and then completes development. By the end of the show, she lets go of part of the fantasy world.
The main plots are developed well and so are the subplots, but some of the side plots aren't. Personally, I see it as untapped potential that could be explored in a spin-off since there is also foreshadowing and visible plot hints, rather than feeling like it's a random thing thrown in.
Overall, the show is really good.