Their specs help, but we also support their old format which was basically a dump. We had to reverse engineer it.
Also, we'll import from Figma this fall. You're right, it's never easy. It's a bug valley of countless files that render a little distorted.
If I were searching for a Figma alternative, its file format being documented would be a priority because otherwise, one is subjecting themselves to some degree of lock-in.
[0]: https://developer.sketch.com/file-format/