For example, I frequently need to save a PDF from my Google Drive app either to my local Files or the Books app. But both those options are just... missing from the iOS share sheet, even though Acrobat is there. So I have to share to Acrobat, then share to Files or Books from within Acrobat. Bizarre.
The problem is some kind of combination of file type plus application -- Gmail will let me share a PDF straight to Files or Books just fine.
Or sometimes when you open a PDF in Books from another app it saves it to Books as a copy... but in other cases it opens it in-place. So you think you copied a bunch of things into Books to read on the airplane... and then they're not there.
The fact that there's never any indication whether something is being copied or not, and that apps that support a filetype are simply arbitrarily missing from some share sheets (but not all), makes iPadOS just a disaster for any serious work that spans multiple apps.