Imperfect, but serviceable and there are no alternatives. – PaperShip - Mendeley & Zotero Review

Navigating your library is not fast, but it's fine. Reading and annotating PDFs is in my experience very robust and nice. (Several reviewers on here say that they cannot handwrite annotate directly with a stylus, but I am doing so with the Apple Pencil 2 on an iPad Pro without issue.) Syncing is slow and not always reliable, especially if you have a large library. The largest problem is that I can almost never get it to add new items to the library. It just silently fails. I have to add everything through the Zotero desktop, or in a pinch I use Zotero in the browser. Another feature it's lacking is a PDF table of contents viewer. navigating long PDFs is therefore slow.
Review by PhysicsBus on PaperShip - Mendeley & Zotero.