Currently, cvs import ignores module configs and, in the future, svn imports will ignore externals (instead of failing when externals are used, bug 81684).
That means that imports from repositories that use these feature will be surprising because checking out the bzr branch will yield less data than checking out the cvs or svn branch it was imported from. Generally, such "incomplete" imports are not be able to build.
Transparent support for cvs configs or svn externals is possible, but it is not expected to be implemented in the foreseeable future.
When an import ignored a cvs config or svn external, the web page should report something like "The following externals definitions have been ignored: ..." so the user has the information to explain why the import appears incomplete, and to be able to build the complete nested tree.
Assigning to me as it is a code import problem.