Okay, well I can tell you that it's generally a valid test. What may be invalid is administration of the test or interpretation of test results.
Someone who is well-trained and knows what they're doing would be well-aware of the possible alternate explanations that you've mentioned (losing interest in the test, fatigue, lack of attention, etc.). The problem is that a score is a score, and any interpretation of why someone obtained a score gets very subjective. Like I said, all one can say is what score someone obtained, which represents how they did to the general population of others the same age.
I'd also be interested in hearing, as someone else suggested, if there's a pattern of strengths/weaknesses on the test that make sense, like a strength with verbal information and memory but weakness in perceptual reasoning and processing speed. Scores that discrepant are very rare, but who knows.
Did you go to a reputable place to get this done? Was it done at school? There are so many unqualified people out there doing this stuff. I'd also second the concern about how much evidence there is out there about biofeedback. Learning and school stuff is not my specialty, but in my field, biofeedback is generally regarded as a joke. I don't know if it differs in other areas, though.