Start with the product constraints
List offline behavior, notifications, hardware access and third-party SDKs. Verify support for the actual features before deciding how much code can be shared.
Understand what remains separate
Even a shared codebase has platform-specific permissions, builds and store releases. Include checks on both iOS and Android in the plan.
Consider the people maintaining it
Existing skills and dependency ownership affect the continuing cost. A framework advantage can disappear if the organization cannot support its native integrations.
Prototype the uncertain part
Test the most difficult device or SDK requirement early. Use that evidence to choose an approach rather than relying on a general claim about speed or performance.