Arenas let you use OOBAs to do data attacks, and those can give you the moral equivalent of remote code execution.
Like if your arena contains both a buffer that OOB's and a buffer passed to a syscall, then having memory safety around the arena doesn't help you
Yes. This is a big headache in graphics, where there are big tables indexed by texture index down at the GPU level. Managing those as the content changes, with the GPU using the data while the CPU alters it, leads to huge complexity in Vulkan graphics.
The two times I've had to deal with a really tough debugging problem in Rust, it's been related to some crate which did that.
And as a bonus, they also conveniently opt you out of any other hardening features your system malloc might have implemented (like MTE).
I certainly appreciate the performance benefits of custom allocators but it's disheartening to see people naively adopting them just as the world's allocators are starting to get their acts together with regards to security.