Syncing itself is simple, certain tools that do it are not.
rsync is a tool for people who write scripts. You may feel a lot more comfortable with e.g. cronopete, or BackInTime which is powered by rsync under the hood.
Everything depends on how comfortable you are relying on a certain level of security... but I would advise storing all files in one cloud storage provider, and including that folder in a snapshotted backup program like cronopete or anything else on the list I linked before.
Chielfy, the backups are there to save you if you accidentally delete something and don't notice for a while, or files get corrupted.
Cloud Storage will sync the corrupted version to everywhere!
It happens.
Someone who is very careful about his data should make doubly or triply redundant backups. Not everyone is careful, and very often they escape accidental disaster. Sometimes things happen.
