What makes a musicals book good or bad?

Hey yall! I'm an aspiring musical theater composer/lyricist, and I've just recently gotten into analyzing shows more deeply. I think it's an important skill for what I want to do. I'm not having as much of a problem with accessing the good things in a show, but it's the "bad" that I'm struggling to pick up on. Maybe I'm just not critical enough or I haven't had the practice to hone in on those crucial details?

Jekyll and Hyde has some of my favorite music in any show, so I watched the 2001 proshot with Rob Evans as Jekyll/Hyde and I thought it was incredible. However, I have read numerous times on this sub and others that the book is not up to par, and I'm just trying to understand why. Besides the show obviously needing to end on Confrontation (in my mind at least), I don't understand the hate really.

This is not just about J&H, but more generally about any show. It's important that I learn what pitfalls to avoid in my own creative process and how to more critically access other's works to better my own.

Thank yall for the read! I truly appreciate this community and hope that everyone enjoyed the Tonys tonight!