I tried logging into an XFCE wayland session and it just dies.  I
checked .xsession-errors and it says it can't find "startxfce4
--wayland" like it is not interpreting the switch as an argument but
rather part of the exec name.  I am not sure if there is a way to
specify command line arguments in the wayland-session file or if a
wrapper script needs to be created.