I've been using XFCE for about 8 years and one problem I had since beginning is how sessions (don't) work (restoring open windows after reboot). I have experienced it not saving session at all, not saving Thunar at all and saving Thunar only if it was started from Applications Menu, seemingly completely independently on settings. I have never managed to debug what was causing it. Currently it works correctly on my work computer and is buggy on my home PC, even though they are both same distro and version and I'm not aware of changing any setting that could affect that.

That said, I consider XFCE great and all bugs I encountered while using it were minor.