Udemy's appeal for PSPO I prep is the price — self-paced video courses covering the Scrum Guide's Product Owner accountabilities, often just a few dollars during sales. Like PSM I, PSPO I doesn't require an accredited course, so Udemy is a legitimate and common starting point.
The gap is the same one that shows up across self-paced marketplaces: a video course can't tell you whether you specifically understand product backlog management, value maximization, and stakeholder collaboration well enough for the exam's scenario-based questions, versus just having watched the material.
Edureify's Diagnostic maps your understanding across those specific PSPO I areas on day one, and the Readiness Score updates as you practice — so you get a specific, evolving answer instead of a vague sense of how far through the videos you are.