If you go for a NAS, this, as already mentioned by the other members, is not a real backup as it won't protect against issues with the power supply let's say.
Relying on portable, mechanical hard drives is also foolish if you ask me. Out of all the storage devices likely to let you down, the portable mechanical hard drive is surely at the top of the list.
Nothing wrong with a NAS so long as it IS used as back up and not the sole storage device. A power supply failing in a NAS is just as likely as the PSU in the computer the external is plugged into failing.... ultimately, everything relies on a PSU.... even externals. You are suggesting that the PSU in a NAS is somehow more likely to fail... which is clearly not true. Are you suggesting that something like your company's very own My Cloud EX2 is less reliable as a means of back up than a 2.5" USB portable hard drive?
ANY device as back up is foolish if it is the ONLY other device with that data on it. The more places your data is, the safer it is, which is where NAS devices score, as they are networked they can be anywhere, and syncing that data can be automatic and scheduled. No back up is totally secure unless a version exists off-site to.
Ideally, a NAS should be used in combination with a second storage device.... so you have a back up of the back up. It all depends on how much you value your data, and what would happen if you lost it. Just ask yourself, "What would be the outcome of losing everything?" If the thought of that is just unthinkable, then surely relying on a single portable mechanical hard drive is just as unthinkable? If you would just shrug and go "That's life", then fine... grab a cheap portable hard drive as that will probably be OK.
However... a single, portable, mechanical hard drive is perhaps the least reliable device for back up so far discussed in this thread.... as the OP has already discovered.