A NAS it's not a backup, it's something you use to store data on it.
The backup I was referring to is the offline one. If I need to backup data, something that unfortunately I don't do as often as I should, I need a filesystem that is reliable, that is proven (ext4 is around since more than a decade, and if we count the previous version even more, so in 20 years I'm confident that I would be able to mount an ext4 hard drive that I forgot in the garage in a modern system, with BTRFS, who knows), and that for it they exist a lot of tools in case something goes wrong (there are ton of tools to recover data from damaged ext4 drives, are we sure that with BTRFS is as easy? If I have a filesystem with compression recovering data I don't think is a simple as running photorec...)
Also, the filesystem of a backup drive is not something you can change easily. I still have an old 1Tb drive that I formatted long long time ago in NTFS, and I never changed the filesystem since having to backup all the data to another drive (find it another 1Tb drive), format the drive, and copy back the data will take 1 day. Not that there are things super important on that drive, mostly is stuff I downloaded from the internet years ago, still it's an example why for a backup drive I don't want to have the cutting edge choice that then creates problems in the future.
Ext4 is ubiquitous, so it's my filesystem of choice for all the purpose that have the requirement that the data must be archived for more than 2 years.
The backup I was referring to is the offline one. If I need to backup data, something that unfortunately I don't do as often as I should, I need a filesystem that is reliable, that is proven (ext4 is around since more than a decade, and if we count the previous version even more, so in 20 years I'm confident that I would be able to mount an ext4 hard drive that I forgot in the garage in a modern system, with BTRFS, who knows), and that for it they exist a lot of tools in case something goes wrong (there are ton of tools to recover data from damaged ext4 drives, are we sure that with BTRFS is as easy? If I have a filesystem with compression recovering data I don't think is a simple as running photorec...)
Also, the filesystem of a backup drive is not something you can change easily. I still have an old 1Tb drive that I formatted long long time ago in NTFS, and I never changed the filesystem since having to backup all the data to another drive (find it another 1Tb drive), format the drive, and copy back the data will take 1 day. Not that there are things super important on that drive, mostly is stuff I downloaded from the internet years ago, still it's an example why for a backup drive I don't want to have the cutting edge choice that then creates problems in the future.
Ext4 is ubiquitous, so it's my filesystem of choice for all the purpose that have the requirement that the data must be archived for more than 2 years.