Sketch has a huge weakness, though: it’s just available for osx, and won’t be ported to other platforms in a foreseeable future. Even worse: sketch files support on other platforms is virtually non-existent.
The world is not just osx, just as the world is not just windows, so depending on who you’re dealing with, your pipeline is severely crippled (export, re-export, figure out which exports are outdated, …).
Adobe would have died a lot sooner if they hadn’t opened up to other platforms. I doubt Sketch can last durably with this policy.