They may be useless, but only because people allow them to be. If you show that you are fully informed about the airline's obligations and the AUC/CAA's obligations to you, you can force them to do their job properly. The advantage is that they will do the work for you for free. They are under an obligation to investigate a complaint properly, and the AUC/CAA have the power to prosecute or bring other enforcement action. They are obliged to act reasonably in discharging this obligation - they cannot simply refuse to help you, or not investigate properly, or ignore the relevant law.

Why not make a reasoned written complaint to both the Swedish enforcement body and the AUC/CAA? Tell them you have complained to the other enforcement body as well, so that they can decide between themselves who should take the lead. Make it clear that you are aware of their obligation to investigate your complaint. Ask them to ask the questions in my post above, and follow the excellent suggestions made by Spotwelder. Tell them that you understand that a simple assertion of "technical reasons" by the airline is insufficent to bring it within article 5.3. It is by far the cheapest and easiest way to pursue a complaint that is not certain to succeed.