I can attest Product Managers were the downfall of one startup I worked at. The company had dozens of millions in funding and managed to piss it all away with ill-founded escapades driven by a handful of Product Managers. If anything, the Product Managers seemed to be managing their careers, not the product. They babied engineers until most of the engineers stopped caring about the product and just gave up working hard at all. All the brilliant ideas were trite insights you could read on two year old Gartner reports -- "release a cloud version".

CEO and CTO beware, do not let Product Managers happen to your company.