How would you decide when NOT to use Swift Concurrency (async/await) in a production app?

You avoid async/await when integrating legacy callback-heavy systems or when deterministic thread control is required.

Although Swift Concurrency simplifies async code, it introduces structured concurrency rules that may not fit all systems. Legacy APIs using delegates or callbacks may not justify full migration. Low-level performance systems or real-time engines sometimes require explicit thread control via GCD or OperationQueues. The key decision factor is control vs abstraction.