What kinds of things does jemalloc perform better in? What are its "weaknesses"?

For example, jemalloc 3.x reduces memory usage of CRuby by ~50% and increases throughput by ~10%. It also makes an enormous difference with KV stores like Redis so it's compiled in by default.

Weakness is basically binary size and code complexity.

Why is jemalloc not included in glibc then?

jemalloc is the default allocator on FreeBSD. glibc uses ptmalloc for historical reasons.

jemalloc is a strong memory allocator. But I expect that the team felt that most applications don't need to ship their own memory allocator.

An issue that Rust has had to overcome is that it produces bloated binaries by default. Hello World type applications are hundreds of kilobytes long, because they ship jemalloc and libunwind. Removing one of those helps.