Microservices test architecture. Can you sleep well without end-to-end tests?

Do you know the rare feeling when you develop a new application from scratch and can cover all lines with proper tests? I said “rare” because most of the time, you will work with software with a long history, multiple contributors, and not so obvious testing approach. Even if the code uses good patterns, the test suite doesn’t always follow. Some projects have no modern development environment set up, so there are only unit tests for things that are easy to test.