What changes at 10K+ daily watch users?
Small sync assumptions become visible product problems. Connectivity drops, stale sessions, duplicate actions, and background timing need deliberate states instead of optimistic happy paths.
How should phone and watch state synchronize?
Give each workout action a stable identifier and timestamp. Apply it once, acknowledge it, and reconcile later when either device reconnects.
The interface should distinguish pending actions from confirmed state so users never have to guess whether their workout was saved.
What should the team measure?
Track failed transfers, reconciliation time, duplicate suppression, and abandoned watch sessions. Crash-free sessions alone will not reveal a product that silently stops syncing.
The useful lesson
At scale, reliability is not an internal quality metric. It is the product experience users feel every time they lift their wrist.