"- library developers can easily see the impact their changes will have on their consumers"

So, to commit breaking changes you need to file 30 Jira tickets first and wait for a synced commit with 30 branches? Or how do you do it? Do monorepo shops work without releases and stable versions?

If dependent projects (in use) pull your head and there is no synced development I assume you will end up with a complete legacy mess to work around other projects unit tests to get anything done.