The one failing that is usually present in these studies is proof of causation. Yes, there's a pretty undeniable coorelation between wealth and happiness, but as has been shown a number of times, if an unhappy poor person wins the lottery, they don't immediately become happy.
It's entirely possible that happiness and wealth just so happen to both be the effect of being successful in persuing one's dreams (which would explain, to a certain extent, the correlation with education). When people go after what they really want in life, they're more likely to be happy. They're also more likely to be passionate about what they do for a living, and that often yeilds better results, therefore more money.
while it's entirely anecdotal, I feel like that's what both my happiness and my money come from - i was lucky (especially for those born into the JW cult) to have parents that supported me in doing what I really liked doing. The result is that I'm reasonably well paid, and bad days at work are rare.