I keep seeing talks and arguments about how the portfolio team should manage the epics for a program. That conflates the issue of project portfolio management and product management.
Several potential teams affect each project (or program).
Starting at the right side of this image, the project portfolio team decides which projects to do and when for the organization.
The product owner value team decides which features/feature sets to do when for a given product. That team may well split feature sets into releases, which provides the project portfolio team opportunities to change the project the cross-functional team works on.
The product development team (the agile/lean cross-functional team) decides how to design, implement, and test the current backlog of work.
When the portfolio team gets in the middle of the product roadmap planning, the product manager does not have the flexibility to manage the product backlog or the capabilities of the product over time.
When the product owner value team gets in the middle (or doesn't plan enough releases), they prevent the project portfolio team from being able to change their minds over time.
When the product development team doesn't release working product often, they prevent the product owner team from managing the product value. In addition, the product development team prevents the project portfolio team from implementing the organizational strategy when they don't release often.
All of these teams have dependencies on each other.
The project portfolio team optimizes the organization's output.
The product owner value team optimizes the product's output.
The product development team determines how to optimize for features moving across the board. When the features are complete, the product owner team can replan for this product and the project portfolio team can replan for the organization. Everyone wins.
That's why the product owner team is not the project portfolio team. (In small organizations, it's possible people have multiple roles. If so, which hat are they wearing to make this decision?
The product roadmap is not the project portfolio. Yes, you may well use the same ranking approaches. The product roadmap optimizes for this product. The project portfolio team optimizes for the overall organization. They fulfill different needs. Please do not confuse the two decisions.