TBH, with Bernie it's mostly worrying about the disconnect from the practicalities of his proposals. I live in BC, where we have something like a 95% renewable electricity grid, but the last 5% is non-renewable for a lot of very good reasons. Putting the goal at 100% seems like a huge waste of time, effort and money, and, imo at least, not necessary to accomplish the actual goal of such a proposal.
Same with 'zero deportations' or 'accept all refugees.' You can have a vastly more open country that is accepting of new people while retaining decent security and monitoring, and proper refugee and immigration services are extremely useful for placement and success of migrants. It's possible to retain an open country with good security with the right choices for personell and the right policy goals. Saying that nobody can be deported is being too all-or-nothing.
But yeah, that's why I don't like Bernie. There doesn't seem to be much room for negotiation with him, and his policy goals seem uncompromising to a point well beyond usefulness. There are elements I can approve of, and things I would like to see done, but instead of picking goals that can work, he's chosen to go with ridiculous.