I expect bun to run almost everything that node runs these days. They have an extensive test suit to ensure that.

Even the complicated NextJS runs with Bun: https://nextjs.org/conf/session/nextjs-bun

Do you have a source for your claim?

Maybe if you start from scratch with a new project, but when migrating an old project it's definitely not a drop-in replacement. I try once or twice per year, but it's not worth the effort when the upside isn't that big.