I have been using TrueNAS Scale for a while but have not used base Linux for my NAS. My opinion is if you're looking for a quick initial setup or are like me and didn't want to install ZFS yourself, TrueNAS is rather appealing, but otherwise it doesn't offer much. It has ZFS pre-installed, gives you a webUI to monitor basic things about your machine, and has fairly easy ways to setup data protection with snapshots and backups with rsync or zfs replication. In the more recent versions it even has Docker apps built-in so you can host some basic things. The downside of TrueNAS is that despite being Linux under the hood, it's a lot more locked down so doing advanced measures is more of a pain and much of their "simpler" UI-based stuff is exceedingly basic, half-featured, and lacks documentation.

The way I use TrueNAS right now is to treat the main OS as mainly untouchable. I don't try to break out of the limits placed upon it. I instead use a "Jailmaker" machine (defunct wrapper script for systemd-nspawn) for all my Docker needs. This way the main system remains more stable. If I have to re-install, then it's a simple config import and my NAS is back to how it was.

I would use the built-in VM tools or the built-in Docker tools for this, but A. they weren't implemented or weren't working when I set this up, and B. I found their setup rather... annoying. For instance, I tried to set up some apps with their previous app system and it required configuration before working and yet nowhere did anyone explain how to configure it so I was wokring blind. No one makes guides for setting up an app in the TrueNAS UI, so the extra layer of obfuscation was just a hinderance to me. Compare that to setting it up directly in Docker, there are a million guides and great documentation for everything I get stuck on. Thus, despite being the "harder" way to set it up, it was easier due to the existence of information about it.

So, looking at it objectively, what parts of TrueNAS do I even use compared to base Linux? Not much. I use the WebUI to accomplish basic tasks such as creating or modifying datasets and permissions, snapshots, SMB shares, etcetera. All the basic things are there and I use the UI for them. But ever since that initial setup I spend most of my time in the CLI adjusting my scripts and Docker config files, creating directories inside the datasets, fine-tuning permissions... I could definitely have gone for a base Linux install as long as I knew what to install for ZFS support, some manner of WebUI, and so on. TrueNAS just did all that initial setup for me, and having a more locked-down OS forced me to use safer methods of installing programs via containers and keeping my install a lot more portable which I plan to continue no matter what OS I use.

This was probably not helpful, but that's been my experience of TrueNAS for what it's worth. Whatever you do, just remember: RAID is not a backup. It is protection against drive faults, but an error in the RAID system itself or the RAID pool's data requires a separate copy of the data stored elsewhere to restore.