The problem
The best product roadmaps start on a whiteboard. You draw a horizontal timeline — Q1, Q2, Q3, Q4 — and start placing themes and milestones in lanes. The spatial layout reveals things a spreadsheet never does: dependency chains become visible as you realize you can't start Theme B until the infrastructure in Theme A ships. Timeline conflicts jump out. The team debates tradeoffs standing up, pointing at the board.
Turning that board into a roadmap document is where the energy dies. Whoever owns the roadmap has to recreate the spatial layout in a Notion table, a Confluence page, or a custom spreadsheet. Every diagram, arrow, and spatial relationship has to be described in words or discarded. Most get discarded.
Stakeholders end up with a roadmap that's less specific than the whiteboard was. Engineers end up with a doc that doesn't capture the 'why' that was discussed during the sketching session.