Backup and Reliability
If your NAS device has slots for multiple hard drives, you have a built-in backup solution ready for work. Most users find it rather convenient, however, storing backups in NAS is not reliable. The thing is when having several drives you are protected from the failure of one of the drives, which can then be easily replaced.
Nevertheless, it will not protect you from damage or loss of the NAS device itself, which can damage all the drives inside it. In case you have only one drive, you will still need to find an alternative backup solution.
When comparing NAS vs. Cloud, the latest includes backups as part of the service you pay for. As when you move data to the cloud you perform some kind of backup already as well as the cloud computing service is responsible for your data not to be lost, in case there is a problem. In other words, until the cloud service company goes out of business, your data will be safe.