I have a project subdivided in 10 modules. 2 of these modules uses the assembly plugin with the built-in descriptor "jar-with-dependencies".
When running mvn clean install (or also only maven install) from the root of the project, I see that every time it is building one of the two sub-projects that uses the assembly plugin, it rebuilds some already built projects, even if they are not dependencies of the "to be assembled" project, nor they have any other apparent motivation to be rebuilded. One of the two project using assembly doesn't even has a single dependency on other project, but triggers a nearly complete rebuild.
One of the projects is a WAR overlaying another war, useless to say that it consumes a lot of time, and gets built 3 times.
I've tested also with -o to exclude possible changes in external dependencies, checked all the dependency tree to make sure that there were no circular or otherwise "crossed" dependencies, changed the order of <modules> in the parent pom to the best build order, checked by hand starting from an empty local repository that building the artifacts (by hand, one by one, commenting out the assembly) in that order satisfies all the dependencies.