How safe is it to only back up on iCloud
iCloud is pretty safe (and designed to set-and-forget), but cases of corrupt or unusable backups have been seen on both iCloud (1), (2) and iTunes.
It is a good idea to have both local and remote backups, and backup often, so in case of a corrupt backup you have a recent older copy.  

Also, local copies are much faster to restore, so ideally do one before upgrading or restoring your phone.