Sanders has amazing message discipline, but yeah, his mainstream appeal is up in the air, at the very least. Also, there's a fine line between passionate, experienced voice of the people, and grumpy, red faced old man yelling at clouds. His refusal to discuss anything but intricately nuanced economic policy in interviews may play well with, like, 8% of the national electorate, and makes him a great candidate for labor secretary, but he's also auditioning to be the most recognizable figure of government in terms of diplomatic relations, social disaster response, and just about every other topic of import that isn't just a nerdy, substantive debate about inequality. The people demand competence and seriousness, sure, but there's other parts of the game he just isn't interested in that can't be brushed off without consequence (but Hillary has her own problems with this dynamic, too). The log cabin myth is something all candidates have to deal with sooner or later, but he's less equipped to negotiate that hurdle than others, I think.
But his lack of office holding surrogates that can play bad cop rhetorically and prep events for him cross country with donors in less than 48 hours is his biggest drag compared to his primary competitor. Big crowds are nice, but big money actually puts you in the oval office, if we're talking real politick, here.
