Dependencies and prioritization
To scale Scrum, you need a single Product Owner with multiple Teams. Working as a single Product Owner requires rules beyond the scope of the Scrum as written. If you have several fully independent product owners, prioritizing the whole Product Backlog can become unmanageable.
Quality Assurance and Release Management
When working on a common product, decisions about the level of quality should be uniform. This usually requires the introduction of standards, often inconvenient for individual teams. Centralized power is also needed for the process of product delivery to customers.
Synchronization (between Teams, and between Business and IT people)
As the number of teams increases, so does the number of cross-team synchronizations. Overhead costs may be excessive or communication efficiency insufficient. Choosing optimal conditions (frequency, composition, agenda, etc.) becomes difficult and requires experience in Scrum scaling.
Goal Setting and Funding
Without a common approach to the Goal Setting for entire product group and each individual team, it is difficult to manage product PnL.