Why should we limit use of PHPUnit to test the features we built only? Some failures are often due to server side configuration errors in php.ini file. My effort here is to ensure that the server always runs under your specific consideration.
I initiate XDebug on demand, via URL parameters. But XDebug configurations in php.ini has to be configured accordingly to accept those requests and obey them. Thus, only the scripts that need debugging or tracing or profiling will request the particular service. Keeping memory and CPU free is also important.