Bernie - Way too divisive. He could've peeled off a few Trump supporters, but most Bernie-Trump people left the Democrat Party for good. Having "black friends" like Killer Mike or Nina Turner never helped him make inroads to the African-American community, and he always deflected racial justice questions into the same old tired class warfare rhetoric. His ceiling was Hillary 2016, and down-ballot races could've been a total slaughter.
Can't say I agree that Bernie couldn't have won (though it's not a guarantee he would have either) but this is a very fair point. Given the ticket splitting we saw, there's a high chance the GOP would've controlled both houses of legislature.