There are a few things that are not so good about being a Product Manager:
1. Getting things done from people you do not have authority over: A PM role is a highly cross-functional role. It requires you to work with many people across multiple groups. As a PM, you have to keep the needle moving on your tasks. So you have to get the work done from these team members without having a real authority over them. It is not difficult given people understand their roles and are mostly willing to help. But it is just those one or two people , who might be super-busy or may not an idea of what you are trying to do or are not interested in helping you for whatever reason, that cause a big hurdle. Then you end up spending a lot of time, explaining a lot to get the ball rolling again.
2. Always moving to the next thing: As a PM you hardly get the time you want to focus on the things you want. This can mean that your pricing would have been better or your competitive analysis could have been more in-depth. Even though you want to do that , with the stuff piling on your plate continuously, you may not get time to do things the way you want.
3. Budgets are not available for good projects: You see a good market potential, you see customer demand and you build a business case only to know that for this quarter, your budget has gone to some other project/product.
There are few other things that are not good about a PM role, but those are more subjective (ex. public speaking, travel). I have left those out intentionally. I am sure I am missing on a few more. I will add as I recollect them.