I look at it this way:
He's relatively unknown because in large part he's so young (38?) He has not been scrutinized politically very deeply or for very long. So he is not known in terms of how he would do long term. (Since he is also so young. That would apply to any national candidate under the age of 40 or so.)
He comes off quite a bit smarmy. He's clearly used to speaking extemporaneously to groups in business settings where he is challenged constantly over details. But that high preparation makes him seem glib. This is functionally a plus that may make him seem suspect to some voters.
He has obvious ethnicity. Nikki Haley is just as Indian but has a more familiar Anglo sounding name. This is a subliminal thing that would affect some voters.
I personally think he has proven himself with his scorched earth approach to campaigning so far. He has positioned himself as a partisan attack dog and he's never going to live that down if he ever wanted to move to a different political role. He's either going to have some role in the next Republican administration or he will not and he will then go back to corporate life.