Yeah, channels are the main pain point. In addition to the axioms being simply weird (because it's an easy set to implement), another major problem is that you're essentially forced to use them because they're the only things that can work with `select`, and that's the only reasonable option for many operations. Especially if you touch other code, like the stdlib.
That and the lack of tooling around mutex usage / concurrency correctness. The race detector is legitimately excellent and every language needs it, but it can only catch races that you trigger in tests/builds with it enabled, and few projects write anywhere near sufficient concurrent tests to catch issues in practice. There isn't even a "this var claims to be protected by lock X, but it is not held [here]" lint, or "this var is atomic but used non-atomically [here]" (though this one is significantly less of an issue with generics, as safe zero-cost abstractions now exist).
That and the lack of tooling around mutex usage / concurrency correctness. The race detector is legitimately excellent and every language needs it, but it can only catch races that you trigger in tests/builds with it enabled, and few projects write anywhere near sufficient concurrent tests to catch issues in practice. There isn't even a "this var claims to be protected by lock X, but it is not held [here]" lint, or "this var is atomic but used non-atomically [here]" (though this one is significantly less of an issue with generics, as safe zero-cost abstractions now exist).