I just came across this article from May arguing that grit research is over-hyped. Essentially, the claims are that (1) the effect of grit is sometimes exaggerated, (2) a lot of other known variables (e.g. the much-maligned standardized tests) are more predictive, and (3) grit is not nearly as novel as Duckworth claims. It was mentioned in the comments on this article, where the author worries that if grit is the key to success then advantaged kids are more likely to be in environments that emphasize it. The problem is that advantaged kids are, by definition, the ones with the greatest access to the secret sauce of success, whatever you think that secret sauce may be. Also, whether we respond to [insert latest success fad here] by seeking to help the poor get access to it or by blaming the poor for not having it depends not on our theory of success but our theory of the poor. If we see them as victims through no fault of their own then compassion will move us to help them develop [grit, growth mindset, whatever the latest trendy thing is]. If we see them as people of low character then we will blame them for not possessing our trait. We don't need a humane theory of success, we need a humane theory of poverty. I've said this before.
As an aside, Duckworth spent some time at McKinsey early in her career. Those people are everywhere.