That's just hyperbole, using some existing element in CDE or LXDE or whatever isn't like using KDE - you still get the interface that CDE or LXDE provides. I do not see how you'd make that conclusion.
And i don't know what sort of experiences you had but at least personally i never had any issues with desktops made out of separate components. I mean, consider that Raspberry Pi OS' default desktop is basically a modified LXDE which is made up of a bunch of independent pieces.
Also TBH what you describe with "doesn't work half the time because some random component is using the wrong set of X atoms or the wrong X resources or the wrong Dbus interface or whatever" doesn't make sense. If an application used the wrong X atoms or wrong X resources or wrong Dbus interfaces it wouldn't work at all regardless of which DE it is used with.
Well they may work in some way but you'll see degraded functionality. I don't think it's hyperbole, every X window manager and panel, dock, etc that I've seen has support for a seemingly random jumble of motif and EWMH hints, and rarely will they support all of them. If you're using the standard Xresources then those might sometimes work in other apps, like Tcl/Tk and maybe FLTK I think supports it, but other things, not so much. And if you want good high DPI support with those apps then forget it.
I cannot remember what the Raspberry Pi default desktop looks or acts like as I haven't used that in forever, sorry. All I remember is not liking it for other reasons, i.e. the DRM/KMS driver performance was too poor to be able to handle a compositor or play a 1080p video. But maybe that has changed recently?
I tried Rasberry Pi OS recently on a Pi 2 so it was still slow (especially since i had a very awful microsd card - i'm certain that 99% of the slowness was due to the card) but the desktop was very consistent overall. Note that at the past they used LXDE with Rasbian but didn't made any customizations at all.
And i don't know what sort of experiences you had but at least personally i never had any issues with desktops made out of separate components. I mean, consider that Raspberry Pi OS' default desktop is basically a modified LXDE which is made up of a bunch of independent pieces.
Also TBH what you describe with "doesn't work half the time because some random component is using the wrong set of X atoms or the wrong X resources or the wrong Dbus interface or whatever" doesn't make sense. If an application used the wrong X atoms or wrong X resources or wrong Dbus interfaces it wouldn't work at all regardless of which DE it is used with.