Zarzuela is in fact a type of opera. However, it is opera without so much screeching.
Musical theatre suffers from two major problems as I see it:
(1) The plots and the scripts can get away with being really stupid. When someone drops his hat and starts to sing about it, that is unnatural behavior and ti makes the actors look like idiots. Some songs fit in pretty well: South Pacific, Evita, West Side Story.
Some are just silly; Guys and Dolls, The Mikado. There is one really dumb song in the Sound of Music. All of Mary Poppins is just dumb.
(2) The style of singing can really get on one's nerves. This is a matter of taste, of course.
I have seen staged versions and film versions of many musicals. The film version has ALWAYS been more enjoyable. Not all were enjoyable, but the stage version always seems rather stupid to me,
Again. it is a matter of taste.