The difference was that those were first term midterms and the incumbent was never unpopular to begin with. Democrats rolled off their success in 2006 in the open 2008, which doesn't make it inconceivable that Republicans could do the same.
Still, politicians more than anybody know how quickly things can change. Hillary personally probably knows that in 1990/1991 it was considered a suicide mission to go up against the extremely popular Bush 41. How did that turn out again? Besides, even if Republicans take the Senate, Democrats could easily gain it back again if 2016 is a good (or even modest) year for them.
