This is not a bad game by any means. I consider myself a metroidvania fan, so giving this game a thumbs down is a little painful. Monster Sanctuary has serious quality of life issues that makes playing it more repetitive than usual, and more boring too. My issue is with the map and level layout: The areas don't feel different enough and the game would be better with more specific landmarks. This results in MANY disjointed questlines, running in circles and sluggish progression within the map. The Monster designs are quite nice, however some could use more work (Knights and Goblin feel out of place). The combat is very interesting and explained well (which is sad considering the lack of polish in the map layout).
If you are more patient than me (probably because you did not play some of the better metroidvanias) this is a must play for sure.