sce's contain all their dependencies, but if a startup script of a package has already been loaded via another sce, then loading an sce that contains that same package will not re-run that script. And there really should not be different versions of libraries as part of different sce's, hence the use of Debian stable.
Let's pretend that both emelfm2.sce and mydesktop.sce both contain the dbus package. If you load emelfm2.sce, then dbus will be installed and it's startup script run. When you later load mydesktop.sce, the startup script of dbus is not run again since it has already been done as part of emelfm2. And if mydesktop.sce contains a newer importing of dbus with a newer and corrected startup script, it will not be run on loading.
This is why I generally use a large sce with all my favorite apps contained in it via a file list. It takes some time when needing to re-import it, but the debs are already downloaded and all it requires is just time and some cpu cycles. It is not often that a low level library has to be fixed and then require the re-importing of existing stuff, but it sometimes happens.