Quick answer. In-app messaging only reaches customers while they’re actively using your app; RCS reaches them in the native inbox anytime, with no app required. In-app messages are powerful for guiding users mid-session — onboarding, contextual tips, offers at the moment of use — but they can’t reach someone who isn’t in the app right now (or never installed it). RCS complements in-app messaging by being the outbound channel that brings customers back, with branded, interactive content and SMS fallback.
Best practice: use in-app messaging for contextual, in-session guidance and RCS for outbound re-engagement and notifications that reach the whole audience.