> Bun seems to use a different runtime (JSC) which is less tested than V8, which makes me assume it might perform worse in real-world tasks (maybe not anymore?).

JSC is still the JS engine for WebKit-based browsers, especially Safari, and per Apple App Store regulations the only JS engine supposedly allowable in all of iOS.

It's more "mature" than V8 in terms of predating it. (V8 was not a fork of it and was started from scratch, but V8 was designed to replace it in the Blink fork from WebKit.)

It has different performance goals and performance characteristics, but "less tested" seems uncharitable and it is certainly used in plenty of "real-world tasks" daily in iOS and macOS.