Hi,
Generally speaking, "effective" means "achieving its defined objective(s)".
What are design validation's objectives? IMO, to establish through sound objective evidence that users' needs, as defined for the given device, are met.
Have you defined and documented what the users' needs are, for the given device? If your design validation provided good (or at least reasonable) confidence that these needs were met, then it was effective by the definition I began with. Risk management is an interacting and completing activity for enhancing your confidence that there will be as little as practically possible consequent warranty failures.
If you're looking for 100% confidence, then I don't think any single QA tool can provide that. I'm not even sure any combination of tools available will guarantee a 100% failure free device (at least not in real-life world). That's why post-marketing monitoring and continuous improvement tools are so important.
Cheers,
Ronen.