There's a subtle difference between "productive" and "easy" in my mind. A productive day is one in which the fish are there, they are feeding, but it still takes a bit of skill to get them. Easy is when the fish are there, are feeding, and anything you do seems to work as long as the fly actually reaches the fish. While I've never known a day to be too productive, I have on rare occasions found the fishing to be too easy. Spawning bluegill, trout "farms" and other situations where either the quantity of fish or an exploitable instinctive response makes the ish unusually susceptible come to mind. On those days I often find myself moving on to explore new areas or lying back and taking an afternoon nap. Thank goodness those days are extremely rare .
If it swims and eats, it'll eat a fly.