When it comes to fast bowling - yorker is the most difficult delivery to bowl and to play as well. Most batsmen find yorkers difficult to play as there is very little margin for error because yorkers swing and dip late making it extremely difficult for the batsmen to pick. Moreover it is pitched right up on the batting crease making it impossible for the batsmen to get under it and hit for runs (except if you are MS Dhoni and you have the helicopter shot).
The ball is full and if directed at stumps, the batsmen is at huge risk if he misses because the ball is sure to hit the stumps or the toes of the batsmen.