> My conclusion is that the favorability ratings are bullshit.

Primary election results do not support this theory. Clinton has very high favorability among self-declared Democrats, something like 70-80 % off the top of my head. Her favorability declines fast with Independents and Republican voters. I don't have the data in front of me but I could bet Sanders won independent vote in every primary where independents were allowed to vote, notably Michigan. Noone can't say if it's favorability only but correlation is hard to dispute.