The question of who makes a better project manager; the person who is a subject matter expert and becomes a professional project manager, or the professional project manager who has limited functional experience in the product of the project?
I have always answered that the professional project manager with limited functional experience in the product of the project makes the best project manager. She will take nothing for granted, her assumptions will be clear, and she will have no preconceived notions of scope, time or costs. She will figure it all out using effective project management tools, techniques and processes. She can focus on inspiration and leadership of her team and not be pulled down into the weeds of minute details of specific product requirements.
This is where team matters. A good Business Analyst (BA) who can truly elicit and manage those requirements is the key. Without that BA, who is the catalyst between the product of the project and the end users, very few projects succeed regardless of the skills of the Project Manager (PM).
Another approach has been taking a single person and making her the BA and the PM which is the usual outcome when the PM is also a subject matter expert of the product of the project. This is a huge risk to a project. Being a successful PM requires more of a "Right Brain" dominance and a good BA requires more of a "Left Brain" dominance. Trying to do both is almost impossible and is the primary cause of burnout in project managers and dissatisfaction in project teams.
Trying to bridge the left and right hemispheres of the brain and performing concurrent tasks that require unique skills that are inherent in one or the other hemispheres creates high stress and chaos in the person attempting the work. The PM tasks will suffer or the BA tasks will suffer, usually it is the BA tasks that suffer in the form of poor requirements elicitation and management (the #1 cause of project failure).