It is hugely disappointing to lose the immense functionality and customization capabilities of the desktop version of OneNote. The app is fine for tablets and phones, but it is does not even come close to the level of functionality the desktop version provides on a full computer. Navigation is slow, with no options for customizing it to fit the user's workflow, there are no options for add-ins to provide missing functionality, and it doesn't even connect well to the other Office programs like the desktop version does.
For those who want the simplicity of the app, let them use the app. For the rest of us, the app is years away from satisfactorily replacing the desktop version. I have accumulated multiple gigabytes of data in my OneNote notebooks, and I am now wondering if I made a big mistake in putting all that irreplaceable information into OneNote. I chose OneNote over Evernote because it had so much more functionality and ease of use. This move by Microsoft will erase all that.