I've had a 4 bay qnap unit for about 3/4 years now. It's been very reliable, and large external (single) drives weren't available at the time, but it was quite expensive and if I was buying again now I'd just get a couple of 2-3Tb external drives and set one to mirror on a nightly/weekly basis. If I was really concerned about my data I'd get a third drive to keep off-site.
With a Mac you have the option of FireWire or Thunderbolt which are both significantly faster than a NAS or USB drive.
Remember no backup solution is 100% effective, even a NAS drive. Multiple drive failures (unlikely but possible) will lose your data as will fire/theft etc if you have no other copy.