With chroma key you have to organize the clips so the clip with the chroma key effect is on top of the clip you want to be visible underneath. It’s fine for two clips, but when juggling 10 clips on top of each other to make a mockup video of a feature it makes it a very complicated jenga.
Edge crop could benefit from being able to target clips.
Perhaps it could end the need to be on top of each other, since only a target is required to portal the effect into.
It would have helped me a lot when I was making this mockup with tons of green screen clips on top of each other and I had to figure out which order.
If I could target which clips additively affects which clips without having to put them on top of each other in a very particular order would make it look much cleaner and easier to use.
You can with compositions, but you still need to pay attention to what the tracks in between are doing. This article is a very good explanation of how timelines/tracks work in Kdenlive.