I don’t know that I have ever heard a compelling reason for a product to not have a unified backlog. Typically, when there are multiple backlogs, you have a lagging indicator of dysfunction in the business operations. It is a statement of multiple top priorities, which is an insight into chaos. Granted, when you have multiple teams and any are Scrum teams, those teams will have Sprint backlogs, but that is different (and mostly fine)
Scrum of scrums is good for tactical and technical blocking and tackling. I also like using product councils or similar for the business side to ensure harmony in the strategy/vision/backlog further out than what is typically covered in a SoS.
I like using the three horizons of planning, leveraging OKRs to fuel backlogs, and making damn sure those that set priority either have the data or the awareness to leverage data when making prioritization decisions. We are well past the “because I am the product manager” level-of-thinking in this industry. That is no longer an acceptable reason to position an idea on top of another.