I do see some issues with really large files. Files over 50Mb. My server has limits imposed and the resulting error is not communicated back to me on the app. The app merely states "Failed to upload". For me, this is the only issue I ever get, but without knowing the cause it does feel very "random".

In my case, I managed to track it down and, by knowing what causes the sync issue, I can avoid it, or ignore it. Could be that grandparent has this issue too bu her/his server is set to much lower limits even?