Frozen fruits, when cooked, tend to release a lot of water, and this can mean a runnier pie filling. Raspberries and strawberries are especially prone to this. Frozen blueberries and blackberries should be alright to substitute, but try to stick to fresh strawberries and raspberries. Since fresh rhubarb has a limited season, frozen can be used if you let it thaw and drain away the excess moisture before chopping it.