Longitudinal consistency of StrengthsFinder
Seems like StrengthFinder and CliftonStrengths are owned by Gallup organisation and there is commercial registration of the tests? The same way as psychometric testing can be commercialised and protected by intellectual property law, it seems that Gallup has done the same with those tests. 

As the result, research and testing into those instruments are very limited and stifled by the risk of litigation by the Gallup organisation. Makes it hard to independently verify and test the assumptions and studies done by Gallup unfortunately.