MS has a standard test suite for WHQL which verifies that API calls are handled according to spec, and that the hardware meets the minimum quality requirements of the API (this WHQL suite is also available to developers, so they can test before submitting the drivers).
So WHQL mainly means that the drivers are Windows/DirectX-compliant. Hence the software-side should be reasonably stable.
Microsoft however does not specifically test things like fan control, as this is hardware-specific. Nor do they bother testing with various games... a lot of games have non-standard behaviour, breaking API-specs. This may cause render bugs on drivers that implement the standard correctly. Usually a workaround is implemented in the driver for that specific game.