So far, so good. In initial testing, I was able to plug into glfw just fine. Libc is available on both macOS and linux, and it has pretty good support for interfacing with C libraries. That provides a good basis for cross-platform development, IMO.

I haven't dug into testing Foundation support yet. From what understand, it's mostly implemented and they're working on finishing it. Also right now they only support 64bit OSes and only have prebuilt binaries for Ubuntu 15.10 and 14.04.