My biggest concern about Pete is definitely that I can't assess how hard he will work for the big changes on which he is campaigning. I'm not saying that he's a liar; he seems pretty genuine. Nor do I think he's weak. I just don't know what he's willing to use his political capital for when push comes to shove. With the kinds of political reform he's pushing, it would be pretty easy to throw up his hands in office and say, "I tried but there's not much I can do," and then play the part of generic corporate Democrat.
There are two top candidates like that for me: Pete, based on his minimal track record, and Harris, based on her consistent obfuscation and contradictions. I know what Bernie and Warren will fight for (and I like it). I know what Biden will fight for (and I like it a lot less). Even Beto, whose policy positions are less distinguishing, I believe that he'll fight hard from the heart. With Pete, all I really know is that I'll be getting a smart, interesting guy who's willing to consider new ideas. Which is light years ahead of Trump, but not enough in the primary.
The other thing with Pete is that he's getting a lot of donations from billionaires, who generally have a strong interest in not making structural changes. Maybe he's found a bunch of really socially conscious billionaires, but I don't know why they'd zero in on him alone.