All the issues I had with Xfce while using multiple displays
First of all I want to say I love Xfce, it is a great DE, but its multi-monitor support is far from being great so I wanted to list all the things that make using Xfce with multiple monitors a poor experience.
1) Panels on top of fullscreen apps
This is the biggest issue by far. When you have a fullscreen app on one of the screens but you are focusing another window on another screen the panel gets on top of the full screen app or video. This is a big issue if you are playing a video or something because the panel gets on top of the video. I have seen many users complaining about this issue and I have no idea why there isn't a fix yet. I have seen someone saying this is the intended behavior, but out of all the other DEs I tried only Xfce behaves like that and it makes no sense at all. The only way to watch fullscreen content while doing some work is to temporarily remove the panel otherwise it will get on top of the content you are trying to watch, this gets annoying very fast.
2) Lack of an option to have workspaces only in the primary display
I believe many users (including myself) don't want to deal with workspaces in all their displays, in many situations having workspaces only in the primary display might be ideal and easier to manage your applications. This is actually the default behavior in Gnome these days and I wish it was at least an option in Xfce.
3) Missing shortcut options
There are no shortcuts for multiple displays available in the Window Manager app. It would be nice to be able to configure shortcuts to move the active window to the top, bottom, right and left displays. Additionally, shortcuts to change the focus to the top, bottom, right and left applications (in the same or different displays) would also be very helpful.
That's it. Again, I love Xfce but once I connect more displays the experience really isn't that great. I hope this can be fixed in the future.