From a laymen's perspective, Medicare for all seems like a dandy idea. For people who understand healthcare, it would be an abject disaster.
1. Taxes - the revenue for universal health care needs to come from somewhere. It will be in the trillions of dollars. One can argue to siphon funds that were earmarked for something else (e.g. military funding) vs increasing taxes on all.
2. Hospitals would rapidly go under financially - it is will known that acute care centers lose money every time they see a Medicare patient (let alone Medicaid). They stay afloat by gouging HMOs and PPOs 300-400%+ of what Medicare pays. I'm not a fan of hospitals buying up private practices, monopolizing markets, and driving up costs - HOWEVER, let's be serious, we need acute care centers.
3. Health plans like Kaiser who do a good job at turning profit, rationing care, and limiting health care expenditure would go under, assuming there was a universal mandate.
4. Politics - there is zero chance that Sanders or any other candidate to pass this proposal through Congress.
There are more, but the above is a good start. I don't want to make it seem that I am a passionate advocate for the current system - I am not. However, we need baby steps to get where we need to go. We can't throw the proverbial baby out with the bathwater.