I feel like gaming is the easiest thing to do cross-platform since you don't rely on any native UI and OpenGL is pretty similar on different platforms. Unity, Unreal Engine, and many others are designed to target many platforms.
Also, in regards to cross-platform frameworks and games: it's the additional latency, as well as the overhead (but almost certainly the latency) of any cross-platform compiler/library/framework that kills things. Games are right on the edge of a 12 millisecond per frame boundary (or less!) and every cycle usually counts for games these days (especially on limited mobile processors).
You say that, but plenty of games, including twitch shooters, have been successful using Unity and Unreal, both of which support a pretty large number of platforms.