"The hard bit about good synopsis critiques is that people don't see all the shiny bells and whistles. They don't see your story. They see what's in your heart. They see what your story secretly wants to be."
I've only recently been through in-depth critiques through synopsis and it's really interesting! Novels can be such a marathon and synopsises can really show vulnerabilities as well as disconnects between what you want your story to be and what is on the page.
They also make you aware of how little cues can send people in different directions as you have to make strategic choices around what you give to the reader. They can be frustrating, scary, weird, and help you dig into the guts of what you're doing.