Flutter is the most popular cross-platform mobile SDK
Flutter suffers on iOS because it completely takes over the render surface (ie. it renders everything using Skia), and doesn't offer any escape hatches for native components. In a way, this is an amazing approach: it completely sidesteps a lot of the platform-specific woes that plague React Native, and makes it much easier to support completely new platforms (ie. if you can get Skia to run on it, it can probably run Flutter).But particularly on iOS, the native components have some incredibly subtle behavior (eg. the "fling" gestures from UIPageViewController, the native modal animations, overscroll, etc.), and these are extremely difficult to get right.React Native's story for native inter-op has recently become miles better with JSI, which allows nativeJS FFI at much better performance than the previous approach. This has enabled libraries like Reanimated 2 (https://www.reanimated2.com/) and Gesture Handler 2, which take full advantage of this and get React Native apps tantalizingly close to "truly native"-feeling.