Make the game too easy and players will complain. Make the game too tough and players will complain. It's hard to calibrate the difficulty of Android games because you have such a large audience. You can't tell whether your game will be played by a hardcore gamer or by a casual player mom.
If the game is too difficult then it will frustrate the user and if it's too easy the gamer gets bored. Both cases have the same result: users quit playing our game. I wish there was a way to custom tailor the difficulty for each and every player... The solution is adaptive AI!