In terms of backup, I don’t think it sound. But it’s better than nothing.
Putting pictures in the cloud can add flexibility to a workflow. It’s a wonderful technology. And for a business use, it is probably good enough as a backup because business records have finite retention periods and business can buy operational interruptions insurance.
But your pictures are not fungible. Money can’t replace them. They don’t have a short retention period. And pictures in the cloud are not under your control.
Miss a payment, they are gone.
Catastrophic data center event, they are gone.
Change in the host’s business model, gone.
Your account compromised, gone.
Plain vanilla ordinary operator error by you, gone. And these are more likely with the cloud because you will be touching the storage all the time for ordinary operations not just backup.
A good backup strategy is the opposite: the backup is offline and read only and redundant. Tactically, backup is based on reducing failure modes and creating multiple paths to recovery.
The cloud can be a convenient skirmish line. But it’s no substitute for hard disks in safe deposit boxes. And another with a family member in another town. And so on.