They can slow the iteration that’s at the heart of creating great products. If the same person is designing and implementing, the feedback cycle takes place within that person’s head. Even if product managers have equally good ideas, they’re likely not able to experiment with as many of them.
This is something that I agree with having been a PM. That said, it is crucial for PMs to not accept this and actually move that much closer to hacking. I have a strong development background and I always make it a point to understand the code, deeply. So that it is easy for you to contribute, relate and most importantly prototype ideas in your head.
Also, one thing that I try and practice, is be part of a sprint and work on developing a feature end to end. It might slow you down in your perf and PM requirements to be promoted. But it is a significant investment for the long run. Code, always. You'll be a better person for it.