You've probably heard this before but there are a lot of people who'd be interested in a cross-platform (NAS/Linux/BSD/Raspberry Pi) version with a command-line or web interface.
As is it's really best for workstations rather than the NAS/servers a lot of people who care about their data use (not just basement-dwelling data hoarders but also professionals like photographers and videographers).
I currently use Borg on my servers, and Arq on my client machines.
I'm quite sure if Arq was available in a command-line version for Linux/FreeBSD, i would use that instead. It has been nothing but rock solid on my client machines.
Restore through the UI is a bit wonky, but once it's started it works well.
My biggest reason is probably that i prefer to use the same (well known / proven) tools across the line.
I don't think Arq brings something to the table that Borg/Restic/Duplicity doesn't already provide, besides a piece of software i'm familiar with, and trust. (and trust doesn't come easy when it comes to backup software!)
As i wrote, i already use Arq on my client machines instead of the mess that is "TimeMachine over wifi", but when it came to backing up my NAS and server/VPS, i had to look for something else, as Arq is not available on those platforms.
My use case would be backing up with Arq from a FreeBSD/Linux NAS/Server to a remote (networked at least) target, and in case of a full restore, i would use the command line as well.
I would then use Arq for Mac/Windows for "plucking" single file restores.
As is it's really best for workstations rather than the NAS/servers a lot of people who care about their data use (not just basement-dwelling data hoarders but also professionals like photographers and videographers).