What are you top reasons for recommending people to NOT be a PMM?

While I certainly love product marketing, it's definitely not for everyone. PMMs often deal with a fair amount of ambiguity. Even if the problem is clearly defined (which isn't always the case), there can be many different paths to solving it. It's on the PMM to create the structure to solve the problems at hand. If you prefer clear cut problems and solutions, PMM might not be a fit.

PMM can also be a high visibility role. You need to be the champion of your area, whether that's internally to different teams, externally in the market, or both. You're going to be the person many different teams come to when they have questions or ideas. If you prefer executing in the background, PMM might not be a fit.