There are a lot of reasons to be wary about a Medicare for All plan:
1. The cost... people will point out that total healthcare spending is projected to go down, but it's not so simple to "move" the money that's spent in the private market today into government coffers to spend on M4A. It's logistically difficult but moreso it's politically difficult. People don't like taxes more than they don't like paying their employer for a benefit. There will also be people who pay more in this new tax than they paid on their old employer plan, they won't be happy either.
2. A main reason that overall costs would be projected to be lower is that Medicare has WAY lower reimbursement rates for providers. M4A basically represents a mass defunding of hospitals and providers. If you look at healthcare professions in other countries they are all paid way less. Big salary cuts for doctors, nurses, pharmacists, physical therapists, respiratory therapists, x ray techs... you get the idea... is not going to be popular. But you're forcing hospitals to take massive revenue cuts with these reimbursement rates so what else is there to do, labor is their largest expense.
3. Complete government control of what is and isn't covered, how everything is administered, etc. It's Sarah Palin's "death panels" but real this time lol. Obviously we have a ton of people on government plans already, and government run healthcare is fine if it's run by competent people who want to see it succeed and know what they are doing.
The problem is that our country has a political party hell bent on making public goods and services worse and now they're full of cranks and kooks like RFK. I would much rather a system with private insurers where I can shop around vs. being forced onto a government plan run by a nutjob.
4. Most people like their current insurance plan, most people are covered by insurance, and we don't need to do this massive disruption to the system to cover the people who are currently uninsured. We can expand the safety net, offer a public option, lower the age to buy into Medicare, etc. There are plenty of "boring" reforms that would accomplish the goal of universal healthcare, they're just not as flashy or sexy as Medicare for All so a lot of people don't care because it's not actually about healthcare for them, it's about politics.