You should really have at least 2 backups. One being offsite (cloud). The NAS backups are nice but there are still some scenarios where the NAS box could become useless. A fire or tornado, the NAS RAID controller or mainboard can still fail, no disk redundancy (if setup this way) for example.
Depending on the amount of data you have you would have to gauge how to accomplish this. Some data may not be feasible to backup. For example I have worked with a
sign company locally that I built a 7TB Synology NAS for them and they are currently expanding it to 10TB. The is RAID 10 and contains all artwork and local files. This isn't really feasible to backup to the cloud over their internet connection. So you'll have to assess the risk for your own business.
My own personal setup is an external HD that is backed up nightly (my hot backup). Then I use Crashplan to backup everything to the cloud. I have around 400 gigs backed up to the cloud.