By law, Advantage plans must offer the same coverage as original Medicare. Check at Medicare.gov for comparisons between Advantage plans and original Medicare, and what the law requires of Advantage plans.
I have also heard some stories about people's struggles with original Medicare. Many people also believe Medicare covers everything, but it does not.
Advantage plans are a good fit for many people. You have to carefully research whether or not a plan will work for you, but to label all Advantage plans as bad is not realistic or accurate. And, of course all Advantage plans are not the same in how they deal with the people covered by them, providers, etc. Some are excellent, some are not. And, Advantage plans can be a very good choice for those who cannot afford a supplement, because original Medicare can leave you with big bills because Medicare doesn't cover all the cost of medical care, but only a percentage (usually it's 80/20).
I had both hips replaced under my Advantage plan for a total out of pocket of just shy of $1,000, by the surgeon of my choice (arguably the best in my area) and at the hospital of my choice (the best in the area) with a private room. I would have paid a heck of a lot more than that with either original Medicare, or by paying the cost of a supplement. My sister, under the same plan, consistently gets excellent care from cardiologists, pulmonary specialists and hematologists in addition to having one of her hips replaced, as well.
Advantage plans are not intrinsically bad, and are a great choice for many.