I fully believe that if they were playing with the starting guys they would look a lot better. And it's not really difficult to see why. Look at all the shots Dunleavy gets playing with the starters...he's shooting open jumpers all the time. Most of us would agree that McDermott would make those shots. Probably Snell too.
Contrast that to how they're being used off the bench. They're running off screen after screen like they're Richard Hamilton and sort of being treated like the first offensive option by the defense. It's easier to key in on them when it's obvious you're running a set play for them.
Clearly teams want to run McDermott off the 3pt line and make sure he doesn't get free on screens. Where McDermott needs to improve is in those situations where he gets the ball off the screen and isn't open and has to make the next pass, either to the post player or back to the passer. He seems pretty unsure of himself in that situation and since teams will gear their defense towards making that the case, it is accentuated that he's not great in that situation. Korver was a guy who was very good and getting rid of the ball if he wasn't open. Once McDermott learns to do that, he'll be fine. Or, if McDermott is given the option of driving or posting, he could do that too. That's something I haven't seen much of that I would like to.
As for Snell, he always seems afraid to shoot right away unless there's no one within 5 feet of him. Even when he's open he seems afraid sometimes. There have been a handful of drives where he looked great and went up strong for a layup or dunk and came away with points. There have been others where he does the same drive, but puts up a soft layup. Those are the ones he's been missing. Snell also never seems to use that pull up jumper that he was supposedly the best at coming out of the draft (supposedly he led all rookies in pull up jumper percentage). If he establishes an ability to drive, he can use the pull up as a counter move. Now he gives himself more things to work with instead of throwing up a bad shot or frantically trying to find someone else to give the ball to.