What is a backup device? That is a good question.
Technically, any device can be a 'backup' of the data on an original device. But the second that original device is formatted, reset to defaults, stolen, dropped in a lake, or destroyed, that 'backup' is now the only copy of that data and isn't a backup anymore.
On the other hand, a 'backup' device may have greater statistical chances of going bad on its own (e.g. NAS, with any RAID option, or, a USB key, with a much higher chance of failure than other storage mediums). When such an event occurs, there is also no backup of the original data and thus, no backup device.
The point? There is no such thing as a single 'backup device'. If you believe there is, you're thinking about your data wrong.
You can have a backup strategy though.
A second NAS can be used to replicate the data from another NAS from one location to another over the internet (this is a backup against theft and physical damage).
A set of USB drives that are rotated weekly, monthly, etc. with one or more being held offsite is also a good backup strategy.
Having consistent and periodic testing of the validity of those backups is also important. Many examples of backups being done religiously and when called upon to restore that data, discovering that there was none to restore from the 'backup' device.
My main system (ThinkPad) has three main SSDs: main OS and data drive, Backup, and Archive drives. Depending on the importance of the data I'm working on, a copy of that data will be copied to each drive (always from the original source; the main OS and data drive). This original source data is also copied to my main NAS on a daily basis. And to my second and third NAS on a weekly and monthly basis too.
This is in addition to select data being continuously uploaded to OneDrive and/or pCloud.
The most important data from the three NAS drives are also copied monthly to 4 external drives (rotated, with one being kept offsite always).
The notes I keep on these backups are my guides as to which data in those backups is the most current and relevant (if needed).
This backup strategy affords me many catastrophes worth of bad luck to happen, and not lose any data. I also randomly test files a few times a week from all backup locations to have some confidence in those backups too.
I don't back up my phone because it doesn't contain anything important (I don't trust it to). Yes, when I buy a new phone, I configure it from scratch.
When all backups have been performed, I have up to 19 copies of my data, in five or more physical locations, and along with my notes, I can get almost any version of that data I need. And if I'm truthful, I still don't feel my data is safe (100%).
Any storage device can be a backup device. But that doesn't mean you have a backup.
The strategy for your backups is more important than the devices used. Multiple backups, in multiple locations, using multiple methods/hardware. This is what I have found to work consistently, and reliably in real-world use.
Anyone 'device', NAS or not, can be part of that strategy. But each method I use is chosen for its reliability, scalability, and dependability too.
Any one method on its own has too many failings and shortcomings to be considered a 'backup'.
Depending on how important your data is and how much data you have to preserve, the strategy you create will be dependent on it wholly.
You need to ask yourself what would I use it for?
