...is the self-righteousness.
People who surrender to ideologies often are holier-than-thou. But the world dictates that principles and rights compete and the best solutions involve thoughtful compromises. You can want less concentration of wealth without wanting equality. You can acknowledge the costs of large-scale immigration without being against individual immigrants or less costly levels of immigration.
Clinton is a pragmatist and an administrator rather than a visionary. This is what I think recommends her in this election.
I suspect that when confronted by facts which don't support his principles, Sanders would choose his principles. This is why his arguments sometimes seem to contradict the reality under his nose.
It is surprising to me that anyone who lived through the 1970s as an adult could still be a socialist. Just as it is surprising to me that anyone who has lived through the last 30 years could not have noticed that low taxes don't create a worthwhile trickle-down effect.