As Lanthas mentions, the issue with flashcards alone is the idea that you can just read about a grammar point and instinctively understand it. Attempting to use it, and checking how you did, is one part of the puzzle. Seeing the grammar point in lots of different contexts (not just the same example on the flash card over and over) is another.
Flashcards are an option for reviewing what you've already learnt, but imo shouldn't be used to learn the material in the first place.