Last major one I worked on that needed to be cross platform was a GPS navigator, which had massive functionality in C and C++ under the hood, called from the platform-specific UI/event handling code through Objective-C-to-C++ on iOS, through JNI on Android, and directly on PC native platforms. It worked pretty well. The ratio of platform-specific to platform-independent code was very small, so it was not like writing the app N separate times. Major engineering savings AND each platform's build had its own UI idioms that users of those platforms were accustomed to. Guess I never realized this was a controversial approach to take!