Well, at least they have a somewhat limited scope to work with — there haven't been significant WinAPI updates for more then 10 years, unless you include things like modern DirectX.
I do wish they worked differently though. They're trying to build the entire thing from scratch all at once, but it would've made much more sense if they took a Windows installation and started replacing Microsoft components one by one with their own until there are none left. This would guarantee 100% compatibility with everything imaginable, because you get complete integration testing for free, and probably won't break any licenses either. Didn't the Haiku project use this approach with BeOS?
I do wish they worked differently though. They're trying to build the entire thing from scratch all at once, but it would've made much more sense if they took a Windows installation and started replacing Microsoft components one by one with their own until there are none left. This would guarantee 100% compatibility with everything imaginable, because you get complete integration testing for free, and probably won't break any licenses either. Didn't the Haiku project use this approach with BeOS?