I also see great potential in it for game UIs. Scalable UI with crisp text and vector shapes has generally been a pain [for me] in gamedev with other frameworks.
Best of all I generally have to write most game UI views just once for iOS/macOS (because they won't use native widgets anyway) with a few #if differences.
Self-plug: My pet project's an open source game engine [0] that lets you use SwiftUI over SpriteKit/Metal views. Once I figure out how to write custom DSLs in Swift, I hope to have an API for describing game scene entities and their components in a SwiftUI-like declarative syntax, or maybe just make them all behave like SwiftUI views + view modifiers.
You'd also be able to use mostly the same code (even for touch and mouse input) on both iOS/macOS without needing to use Catalyst (which brings bugs of its own) :)
Best of all I generally have to write most game UI views just once for iOS/macOS (because they won't use native widgets anyway) with a few #if differences.