GUERRRERO I think rather than that. It would be better to add a bookmarking function. This serves 2 purposes. It could mark your last read chapter on any book if you bookmark it. So instead of working with sync which you could accidentally cancel, you could also work with a jump to bookmark feature. The 2nd purpose is that It can flag a specific chapter as something important or valuable to the reader. Let's say a really funny or romantic scene that really caught a reader's attention. That way, any reader could go back to a specific special chapter they want to reread.
Bookmarks won't be limited to just 1 per novel. There should be a way to have multiple bookmarks in every novel. I think that could work, but I may be wrong.
My concern with the sync button on apps is the question of who guides who? PC? Phone? Tablet? Given that each reader has multiple devices, a possible issue may arise in syncing if you do it wrong and you might throw your whole library out of whack because of it. That's why I don't support your idea.
I read on more than 3 devices. 2 computers, 1 tablet and a phone. So 2 apps and 2 browsers. And I have already made some mistakes when being asked to sync a book with the reading state of an older state device. Imagine that happening to all your books? So I don't agree with a master sync button because the leading device info might be flawed at a certain point in time.