Overall, all of them are fine desktop environments.

But the thing is, if you're making a large application, for Gnome for example, then you can't by default be confident that your application can work very well on other desktop environments like KDE or XFCE.

I think this is what the speaker have meant: Linux Desktops moves many responsibilities on to you (You need to test your app against for each desktop environment for example). And sometime, that many responsibilities can be troublesome (burden, to say the least).