It is commonly accepted that any files stored on a personal external hard drive should also be backed up elsewhere (in at least one other drive/location) since external drives can and do fail and having >=2 copies greatly reduces the chances of important data being lost.
Is the same measure necessary for important files stored in iCloud?
Knowing that any large company's 'cloud' storage would likely be much better and safer than someone's personal external hard drive helps, but are they still susceptible to failure? Do files get backed up within iCloud, that is, are there multiple copies of the files in iCloud in case any one of Apple's datacenters gets destroyed?
The basic question I am trying to answer is: is storing important files in iCloud enough, or should the extra precaution of storing important files elsewhere (e.g. google drive, an external hard disk, etc) also be taken?