The 10,000 hour guideline is very valid
Can you cite a source for this claim beyond a self-reference to "Outliers"?
What Gladwell and Ferriss
really share in common is that they're good writers who know that most readers won't question the validity of their claims if those claims are presented as being 'scientific' and well-researched - regardless of if any research was actually performed.
Neither man is any kind of scientist - social or otherwise - and neither has done much in the way of substantive research to back up their claims. Ferris has done none as far as I know. Gladwell is essentially a tomb robber of old sociology papers - which is an especially poor position right now since both the sociology and behavioral psych. fields are undergoing a crisis of validity right now (huge publish bias, tons of data that can't be replicated, many cornerstone studies being found statistically invalid, etc.).
The prevailing business model seems to be:
1. Come up with a vaguely, generally true sounding hypothesis ("practice makes you better at stuff"). Then dress it up with some hard numbers that make it sound like a rock solid scientific law.
2. Cherry pick a few anecdotal examples that support your hypothesis.
3. Claim that your examples constitute a universal truth of human behavior or development. Don't provide data. Keep things pithy.
4. Publish and profit.
Yet, somehow, both these 'theories' have managed to persist in the public consciousness. Maybe when we humans learn to use more than 10% of our brains we'll be able to move on from self-appointed armchair sociologists.