From this bug, reports a bug on using abstract compact classes with abstract methods. This is perfectly usable in GObject derived classes or GType based, but no for compact classes, they are not any of above.
Because is possible to add pointers to a struct, as GObject does, I’ve managed to create a patch to add support for abstract methods in compact classes. They provide pre-defined implementations with warnings outputs, as for GType/GObject based classes, and is possible to override them in derived classes.
Compact classes are really cheap on resources and construction, you can create really useful and easy to use C API from them and can be introspectable if you define methods to access your struct members.
If you want a fully and friendly bindable C library written in Vala, I recommend you to use non-compact classes.
This should start to demystify an old argument that Vala is just for GNOME. Vala for for GObject and GType based applications, no matter it is targeting Linux, Windows or OSX. Its productivity is great and no other language, to day, have same level producing GObject/GType based applications without overhead.