There are two flavors of TrueNAS - Core and Scale. Core is basically a FreeBSD distro and Scale is basically a Linux distro. They're both a base OS with the typical packages anyone would need for a NAS, with sane defaults + a user-friendly web-based management system.
The upsides are that it's plug-and-play for anyone who doesn't want to research all the options available and figure out the various pitfalls on their own.
> no need for SSD for base OS since running off of RAM is rather easy on Linux
I don't understand this sentence. You're running off a RAM disk with no boot drive? What if you have a power outage?
> samba can run in a container thus a bit more control security-wise
Core supports FreeBSD jails and Scale supports Docker so you could run samba in a container on either if you're willing to do set it up yourself.
> server may run something else as well
As before, both have jail/container functionality. I haven't used Scale myself but Core comes with a bunch of "click to install" jail options for stuff like Plex, ZoneMinder, etc. Our machine also runs a Windows VM (ew) and a Wordpress install in a Jail