Usually "unit tests" should be for verifying error code paths. These are valuable at the library/package level and maybe at some level of internal integration.

Usually "unit tests" should be for verifying error code paths. These are valuable at the library/package level and maybe at some level of internal integration. 100% code coverage is likely useless most of the time. At the application level, testing user inputs to user expectations is highly valuable (perhaps more so than unit tests in a lot of contexts). It is this integration/acceptance testing that any application with users should have under automation. Any codebase lacking that risks breaking or regression at the cost of user experience (which, depending on your app might be a fair trade off).