Biggest problem with Sanders, and why I would prefer Warren to him. His uncompromising nature and pure idealism will make him a very unsuccessful president IMO. There is a reason why he has sponsored only 3 bills in the senate (2 of which were renaming post offices).
I believe that Warren while not having as good policies as him (though more concrete), has a better chance on realizing them than Sanders. Biden obviously will do most of the policies (which aren't good in the first place), but he will like to meet Republicans in the middle, and the likes of McConell would totally manipulate him (just see how Biden fecked Harry Reid despite that he had the numbers, but Biden wanted to please Republicans). I believe that Biden will be a continuation of the system, Bernie will be not-influential and Warren will be somehow in the middle.