I think part of it is that with Unity, there are never overwhelming amounts of options on the screen. Once you understand that Super Key = Search for Application, that's pretty much all you need to know.
In the past, I think the "Start Menu" style application-chooser would have been overwhelming for her - I know it was sometimes for me. I'd click then start reading through the menus "Uhh, wait, what did I want? Where is that again?"
Even with the occasional Unity glitch (which doesn't seem to be an issue at all in 12.04) her system is still much more reliable than ever.
I was really impressed and excited to see that Linux was at the point where she could use it with very little coaching - so, worth considering that much at least.
And even if Unity is doing some things wrong, it is well worth refining this launcher concept. Flying up to a 'File' menu to (say) exit an application, or Edit to search a file, is about as stupid as dragging a floppy disk onto a garbage can. Harder to decide exactly what to do instead.
I'm also quite happy that FINALLY some GUI designers are putting elements like quick keys and fuzzy searching at the foundation rather than kludging them on to an interface which is fundamentally hostile to people who have specific actions in mind and/or know what they are doing.
For now, though, the devil is in the details...
Our perception of user interfaces is very much tied to habit - what we're used to. New interfaces take some time to adjust to.
I know some people who hated Hipmunk and the iPad because they couldn't figure out how to use them.
You should give Unity another try. You may still not like it, but if you're a programmer who likes to use the keyboard more than the mouse, you will at least like the HUD.