Some bitcoin wallets use a lot of hidden keys inside. If you only backup the private key of the visible bitcoin address, much of the money may not be recovered from the backup.

Any online backup is vulnerable to theft. Even a computer connected to the Internet is vulnerable to malware. Therefore, it is a good security habit to encrypt any backup that is exposed to the network.

Single point failure is not conducive to security. If your backup is not limited to just one, bad things will make it less likely that the wallet will not be restored. You can also consider using a different storage medium, such as a usb drive, paper, and CD.

The wallet needs to be backed up regularly to ensure that all recent bitcoins and newly created bitcoins are included in the backup. Soon, however, all applications will use a wallet that only needs to be backed up once.