Like everything else they have their pros and cons.
My one slide is only 18" deep and has the same white rubber roof as the rest of the camper. I have a brush on a long handle (made to clean ceiling fan blades) to brush off any debris that may be up there before I close it. Haven't brushed off more than a few leaves yet (haven't camped under trees yet, either). Somebody once posted that they had a problem with birds trying to nest under the slide awning. Another reported the head of their bed was in a slide and the awning reduced the noise of rain significantly.
So, you really need to look at your particular application to determine if a slide awning will benefit you.
Do you have deep slides? Awning flap may be an issue in that case.
Do you camp a lot under trees? An awning may save your seals when you close the slide.
Does your slide have a sky light? Maybe you don't want to cover that up.
Lots of things to consider. Good luck!