Adding a QDockWidget to a non-allowed area by mistake does not fail.
As the dockwidget remembers where it was docked the last time, the user can still undock and redock the widget using double click to unallowed area, but moving the dock with the mouse only works with the allowed area.

Ex:
setAllowedAreas(Qt::TopDockWidgetArea | Qt::BottomDockWidgetArea);
addDockWidget(Qt::LeftDockWidgetArea, dockwidget);