I think the non-electability quotient is less for Warren than for Sanders. Both have very left of center views, but Sanders has the (unfortunate as it is that this exists) extra hesitation factor of being Jewish and a self-described Socialist. (Not that there's anything wrong with those things. I'm just sayin'...)
In my opinion, I believe that in different cities, including other nations, citizens have had little hesitation to elect a Jewish "mayor." The reason being that the problems that mayors get involved with are local, and can get solved with good planning, financial or otherwise. However, the scope of problems that heads of state deal with are just too diverse to make many people comfortable with someone that has a background that is seen as fairly different from the majority of citizens. Meaning, the stereotype for Jews, by many people, is that they tend to "hear a different drummer," or at least show an independent thought process that still does not necessarily dovetail with the thinking of the masses. This should not be offensive to Jews, since it was the reason they didn't become pagans two-millenium before Christianity. It might just show non-Jews that they are more fractured/divided than they might think. Remember, President Obama still was a member of the majority faith in the U.S.
The hesitancy then does not come, in my opinion, from Bernie being Jewish, but for the belief that Jews are independent thinkers; religion being the most obvious.