> Besides, what's wrong with shared memory?

I generally think that it's bad to share memory for anything with concurrency, simply because it can make it very hard to reason about the code. Mutexes are hard to get right for anything that's not completely trivial, and I find that it's almost always better to figure out a way to do work without directly sharing memory if possible (or do some kind of borrow/ownership thing like Rust to make it unambiguous who actually owns it). Mutexes can also make it difficult to performance test in my experience, since there can be weird choke points that don't show up in local testing and only ever show up in production.

Part of the reason I love Erlang so much is specifically because it really doesn't easily allow you to share memory. Everything is segmented and everything needs to be message-passed, so you aren't mucking around with mutexes and it's never ambiguous where memory is who who owns it. Erlang isn't the fastest language but since I'm not really dealing with locks the performance is generally much more deterministic for me.