They also complained about build times, and in fact the Rust rewrite started immediately after some other drama about Bun being unable to contribute back LLM-written changes to the Zig compiler to improve build times, and the Zig team rejecting them in principle. So while memory issues did become the focus later, I don't think it was the entire story.

Migrating to Rust in a one-to-one translation with unsafe blocks does not make the code any more safe initially. It might provide tools to do a significant refactor that solves lifetime issues with Rust's help, but I haven't seen evidence that they've done that either. They're also embedding a large C++ codebase, JavaScriptCore, so there are always going to be unsafe areas and touchpoints where memory issues could live, and Rust won't magically solve them.