The worry (for me) is that Trump can say and do anything and it doesn't touch him, but if someone attacks back, they are destroyed (Bush and Rubio)
Bernie comes across as a very nice guy that will give you anything, and when Hilary gets somewhat snarky with him, it actually makes me cringe.  (Don't attack the nice old guy)   Also, if she gets the nomination then I am afraid the same thing will happen to her that happened to Rubio and Bush.   
Trump attacking Bernie?  Would it backfire?    Bernie has a way of making everyone else seem self-centered  but would it work on Donald?