Alright so here's the explanation.
The edge lighting works only for some apps like Messages or Snapchat because when you get a normal, basic notification from them, the screen usually lights up. When you get notifications from whatsapp, messenger, email or whatever, you only hear the sound/vibration, without the screen actually waking up. Edge lighting is kinda the substitute of waking up screen.
Also, the glimpse notification app works but sucks for one reason and one reason only (at least for me): when you get the nice & cute floating preview of the message, at the top, it actually says something like "Glimpse Notification: Whatsapp: Mom: hey, how are you?" ( just an example but I actually dislike the 'glimpse notif' being there everytime).