I am by no means an expert, mostly a home tinkerer with a Plex server. I use BTRFS because my Synology supports it and I use ZFS on my Truenas box. I also use SHR with my Synology so BTRFS makes adding and upgrading drives really flexible as my media library grows. BTRFS and ZFS are very feature rich, as you mentioned ZFS is very RAM hungry which can be a limitation for people just looking to get into the server space on a budget. I think the instability of BTRFS comes from the way it stores data, it can get very fragmented. EXT4 in comparison is pretty boring but it works well and if you’re just writing data to store it you might not need the features and overhead of the other file systems. Personally I have no real preference, I like my Synology and I like my Truenas machine and as a hobbyist they both serve me well, and I would take either over NTFS for a storage appliance.

Lots of great responses here, I won’t reiterate what everyone has already explained. The big benefits imo are redundancy using better file systems like ZFS (Truenas) or BTRFS (Synology, unraid), and in general better management of the drives, and data stored on them. These appliances support more robust raid configs as well, so you have a lot less risk losing data. The other big one is simplicity for what you need it to do. Creating an SMB share on a PC using windows isn’t hard, but it’s not nearly as simple as the 3 clicks it takes on the purpose built OS. These OSs also usually have built in solutions for hosting any other apps you may also want to play with. That’s just my two cents.