I think it is highly dependent on field. My field, biology, is basically like flipping a coin. Do you graduate in five years or nine? Will you come out with a golden road to tenure or a life destined to work sad, lonely, post-docs? Flip that coin!
No matter how smart you are, the Gods of Biology are fickle. You may end up with a shitty protein, an assay that only works when the moon is aligned with Venus or cells that only grow when you swirl counter-clockwise.
As much as people like to pretend Biology is science...the sad truth is that a lot is practically voodoo incantations to the protocol devised 15 years ago by a post-doc.
It's one of the main reasons I left. You have to be Smart, Persistent AND Lucky. I'm fine with the first two, but when my career is hinged on luck...well, I started exploring fields that didn't quite as much require blind faith in the universe.
:(