I checked the very maven-enforcer project using the new CLI functionality:
mvn org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-SNAPSHOT:enforce -Drules=requireUpperBoundDeps
and found out there is a conflicting library (org.codehaus.plexus:plexus-container-default). See output.txt.
The tricky thing is that is it a transitive dependency, one with scope "compile", the other is scope "test" (which are not shown in the output). In contrast, Eclipse shows the scope and shaded fonts for the libraries with test scope (see screenshot image), which provide an easier reading.