Entity ownership is an org problem wearing a schema
Federation's hardest questions are never syntax. They're "who owns Product?" — and that's a people question.
Composition failures — and the ones that pass silently
Composition errors that block your deploy are annoying. The compositions that succeed when they shouldn't are the ones that page you at 2am.
N+1 at the gateway — DataLoader won't save you
Everyone knows the resolver-level N+1. Federation adds a second, nastier species: the query-plan N+1, where the gateway itself fans out.