LXF174 discussed various desktops. What it did not do is to investigate how well each desktop delivered the underlying technology to the user.
Sometimes a desktop can make the complex seem simple. Other times the desktop can make it difficult to enable what the underlying systems provide. Indeed, it appears, that sometimes it makes it impossible.
The example I want to use is that of enabling the xkb facilities to use the APL keyboard from the extras part of the evdev rules. I have a particular interest in this keyboard - I added a lot to it following Ted Nelson's original work. However, what I have to say here is, largely, more general and will apply to other usages.
Issue 1: Does the keyboard layout selector include the extra layouts by default. KDE yes. Gnome, LXDE and XFCE no.
Issue 2: Can I get to the latching (while pressed) layout switching facility. For APL where I generally want to switch layouts for one character this is important. For language switching, where you want to type a word or a sentence, it is less important. For any other mathematical layout it is likely to be important. KDE no problem. Gnome - as far as I can tell impossible, or I have not found a mechanism yet. For XFCE and LXDE I could do this but had to hand modify some files.
Note: I did not try Unity but I believe, from others, that it is fine after using a gsettings command to enable the extra layouts.
More detail of what I actually did is posted http://forums.dyalog.com/viewtopic.php?f=14&t=567#p2182