Not if we focus on the inventory puzzles: If you have, let’s say, 3 objects in your inventory then you can combine each of the objects with each of the other two - regardless if you are in SCUMM or in Delores. The designer has to find reactions to all of them.
Yes, you can. 
Have a look at “Edna and Harvey”. Beside that you don’t have that many combinations as it seems, because in most adventure games you have round about 10 objects in your inventory. And Ron wrote somewhere that he would like to keep the amount of objects in the inventory low (if I remember correctly).
And we have another drawback: In a drag and drop engine like the Delores engine, I can’t interact with the objects in the inventory in different ways. For example I can’t “open” a tool box or “squeeze” a plushy to make a noise. (edit: Ah, @Guga has mentioned that already 
)
It’s a punishment for people who don’t think. If the puzzles are well-designed, the player will rarely see those generic messages.
You guys could hijack my thread " The new interface in “Delores”".