These are legitimate concerns. Looking at the history of bun and its current homepage the main point is that it offers a dramatic speed improvement (plus misc. quality of life stuff).

At this point the ball is in your (and everyone else's) court to put these claims to the test. It should not be terribly hard to see if the speedup is worth your while or not, JS surely doesn't lack bloated projects that you can try to build. My own personal blog is a bloated Gatsby mess that takes half a minute to build.

That's the one true part of the experience that nobody can falsify.