I am not sure but I think 'trainwreck of a candidate' does describe Trump at least compared to conventional ones.
Donald Trump might have seemed like a trainwreck initially, but he's defied all expectations. He's actually popular, whereas dull-as-rocks candidates like Marco Rubio or Jeb Bush, the conventional picks, couldn't get anywhere.
I bet that if a Rubio or a Bush got the nomination, they would have done well in the general election where there are essentially two choices. I still want to say Trump won't do as well in the general as other Republicans would have, but there's no way of knowing and Trump defies expectations.