Rich Trouton has found that a custom NetInstall set with your own packages will not install the custom packages. Only Apple’s own packages will install. Even when you sign your own packages with an Apple Developer certificate they will not install.
This also affects custom packages added to COSXIP (Create OS X Installer Package).
AutoDMG uses a different workflow and can still add third party packages.
It is unclear whether this is unintentional or not. However, Mac Admins will have to make plans for deployment workflows without custom imaging or system installation.