Ulysses (and probably Quiver too) can just give you a "pretty view" of your existing folder structure, and since it's all plaintext/MD you can work with your data on Windows/Linux just fine, and there's nothing keeping someone from makes similar apps on the other OSes.

I used to use OneNote for everything, until I moved to Macs... Not only there was no Mac version at first, OneNote STILL doesn't let you import old local notebooks into the Mac version.

With Ulysses/Quiver at least I know that what I write on the Mac will still be readable on Windows/Linux..