Serious answer? XFCE doesn’t support multiple monitors with different refresh rates. So that.

That’s more of a limiation because of X11. KDE and Gnome do not support different refreshrates on multiple monitors as far as I know. Its the main reason why I never used multiple monitors. But on Wayland, this issue is solved. So if XFCE is ported to Wayland, they should also get this support for free I guess.