Happy Unraid user here.
The ability to mix and match drives in the main Unraid Array is of course the original feature and draw. Adding a few TB at a time for whatever leftover money I had after taxes each year is really appealing.
But they've added SSD write caching, VMs, Docker containers, a Docker "app store", and recently ZFS drive clusters (mostly for SSD storage).
It's pretty great and incredibly easy to admin. I presently have well over 125TB of mixed Unraid and ZFS cluster storage in a Fractal 7 XL. It's running around 30 containers, a handful of VMs, Tailscale and literally requires less than 20 minutes a week of system level administration (probably more like 5-10 minutes). Of course I'm spending far more than that messing with the actual apps, but that's a personal problem ;)
It gets regular updates, and I'm sure my uptime would exceed a couple of years except for reboots needed to handle the updates and the occasional power outage. You can ignore the updates of course to min-max your uptime. ZFS has been rock solid on my SSD array.
You can recreate the core array bits with a bit of effort and MergeFS and SnapRAID, add Docker, some VM host system, ZFS and a few other things and you can get Unraid "for free" with a fairly normal Linux distro, but the administrative overhead will be a bit more.
One tradeoff is that Unraid exposes a core set of features for each of these, but you could get to quite a bit more specific of a configuration if you go the regular Linux route. The Unraid devs are slowly adding more ZFS features (for example) to the regular interface, but it takes time. Some more expeditionary Unraid user attempt to use those features more or less at their risk with results reported in various forums.