I'm on Medicare with no supplementals. Wanted to ask about disadvantages of Medicare Advantage PPO.

Was just speaking to an insurance agent about health plans for my mother and I happened to mention I was on Medicare with no supplementals. She said it was a no brainer for me to take Medicare advantage PPO and that the only downside was the smaller pool of doctors. I told her I saw a number of mental health professionals and she did say those are the hardest to find in network. Anyone aware of how much the pool of doctors shrinks from regular Medicare to advantage? Is there anything else I should be aware of? Thanks!