I started using OneNote as my electronic notebook a couple of years ago when I bought the Surface 3. With my purchase of the 10.5-inch iPad Pro, I've been using OneNote with it and the Apple Pencil.
A big difference between the two is that I can use the full Windows desktop version of OneNote on the Surface 3, while the mobile version is the only one available for the iPad. I believe the iOS version is the same as the Windows Store version that I could use on the Surface 3.
For the most part OneNote works well on the iPad, but there are a few annoying nits. For example, I use ruled lines which are added to ever new note I create in the full desktop version, but for some reason does not persist in the mobile version, I have to manually add them every time I create a new note.
The other problem I have run in to is that the iOS version does not support collapsing of subpages. If you have a lot of pages and subpages in a list, you want to be able to collapse them so they are easier to manage.
Finally, inking is not really "native" to the iOS version with Apple Pencil. By this I mean, you have to select drawing mode in order to enable inking, it does not automatically detect.
My feedback to Microsoft is that if you fix these three areas, OneNote will be much better to use on iOS.