I'm on MacOS Sierra, using the latest Evernote Desktop app.
I find that whether a new note is created in a new tab, or a new window is opened for it, Is completely random. I can never predict which will happen when I hit CMD-N.
I'm completely unable to drag a note from one window to another using its tab. So unless I happens to start a new note in a new tab. I can never manage to work with multiple tabs at once.
Window->Merge All Windows is always greyed out for me.