I've been doing Xamarin for about 6 months and I get most of me reuse in services. Sure, manipulating the UI is mostly native code but the rest can be reused such as the DB layer, API layer, and anywhere else you have business logic.

Enterprise mobility mostly; moderately heavy on UI (UI standards in the enterprise are rising, particularly for mobile users), but I generally find the bulk of reuse is in workflow, local persistence/offline storage + data synchronization + security/encryption + enterprise infrastructure integration / cloud & "other" services integration (e.g. CMSs, CRMs), etc.