iCloud is the backbone of a number of Apple services, some more problematic than others. At a surface level, iCloud just handles all of your iPhone backups and syncs files between apps. My main problems have come from three different parts of the iCloud service: iCloud Photo Library, iCloud Drive, and iCloud backups. Let’s start with the problems in iCloud Photo Library, which is arguably one of the worst photo backup services on the web today… More at Lifehacker.
This article covers many of the main iCloud problems and these are sadly genuine. iCloud has been a lot better for me recently, however, but I still don’t 100% trust it.