Xfce is one of the oldest Linux desktop environments. The first version was released in 1996, around 25 years ago. The K desktop environment, before version 4 became KDE Plasma, was released in October 1996. Which desktop environment was actually first is up for debate.
Like GNOME, the KDE developers turned the K desktop environment into something different. As far as I can tell, Xfce still uses the same desktop metaphor it started out with.
There may be Linux distros that use Xfce as the only desktop environment, but I can’t tell you which ones without doing some research. It’s actually included in many distros as an option. Distros call those options editions, flavors and spins.
I have examined a few distros with the Xfce environment: Debian, Fedora Xfce spin, Linux Mint Xfce Edition, Xubuntu (an official Ubuntu flavor). I couldn’t install the Manjaro Xfce spin (I couldn’t install Arch Linux either). The Debian stable branch has an outdated version of Xfce (4.12 vs. 4.16). The rest are using the latest version. I suppose the next Debian testing branch (near the end of this month) will include the latest version.
With Xfce, it’s all about the panels. Debian’s default installs two panels, top and bottom, with the bottom resembling a dock. Fedora’s default is the same. Linux Mint’s default is one at the bottom, made to resemble all the other editions they release. Xubuntu’s default is a single panel at the top.
A Linux distro can be comprised of one or two parts. With one part, it’s without a desktop environment, with everything controlled through a terminal command line. With two parts, the desktop environment is added. Much of the system can be controlled through the desktop environment or the terminal command line. Let’s call the first part the back end and the second part the front end.
With a lot of distros, the back end is the same as another distro with a different front end. If two distros have the same back end and the same front end, what difference does it make if you choose one or the other? After all, Xfce is Xfce wherever you find it. I look at things a litter differently than that.
If a distro is based on another distro, which itself is based on another distro, it’s more likely to be as stable as it can get. Ubuntu is based on Debian. Manjaro is based on Arch Linux. Linux Mint is based on either Debian or Ubuntu. I don’t know about other distros. Regardless, the Xfce edition of Linux Mint is probably better than Xfce on Debian or Ubuntu.
When I started using Linux back in 2011 or 2012, I was using Ubuntu. I went back to using Windows for a few years and when I returned to Linux, it was the Linux Mint Cinnamon Edition I settled on. In my opinion, the back end is way more important than the front end. The front end is a must, of course, because I can’t stand to constantly use the command line for anything other than single word commands.