Honestly it's not; BTRFS has been in my 'that's neat, but it's still got a non-zero chance of deciding to light everything on fire because it's bored' list for, uh, a decade now?

The NAS build is old enough to more or less predate BTRFS being usable (closing in on a decade since I did the initial OS install, jeez) and none of the features matter for what I'm storing: if every drive in my NAS died today, I'd be very annoyed for a couple of hours during the rebuild, and would lose terrabytes of linux ISOs that I can just download again, if I wanted to use Jellyfin to install them a 2nd time. (Any data I care about is pulled offsite at least once a day, so I've got pretty comprehensive backups minus the ISOs.)

I know EXT4 and mergerfs and snapraid are not cool, or have shiny features, but I've also had zero problems with them over the last decade, even between Ubuntu upgrades (16.04, 18.04, 20.04, 22.04) and hardware platform upgrades (6600k, 8700k, 10950k) and the entire replacement of all the system drives (hdd -> ssd -> nvme) and the expansion of and replacement of dead HDDs, of varying sizes (4tb drives to 8tb drives to 16tb drives to some 20tb drives).

It all just... worked, and at no point was I concerned about the filesystem not working if I replaced or upgraded or changed something, which is not something ZFS or BTRFS would have guaranteed during that same time window.