First of all, such backup becomes 100% public through any node to whoever knows the key. This means you need to encrypt all the data and store the key somewhere as once it's on IPFS it's publicly available forever.
Secondly, unless the data is requested and stored a longer time on any other node, the only copy will be on your own server, which isn't very good if that's the server who's data you were trying to protect.
IPFS are good for many things, but backup for private files might not be the best use case as for now. Object storage is likely a better idea, and cheap too.