Sorry to be pedantic, but it's cross-platform GUI that still doesn't have a good, officially supported story.
For non-GUI apps, cross-platform works very well! I've deployed a range of apps to production across Windows, Linux (both x64 and ARM), and Docker on Linux too (also both x64 and ARM) with great success.
I'd like to see an officially supported build for BSD, but that's about my only cross-platform want.