At least one of the ball carrier moves as been ridiculously over powered since they were first introduced way back in the day. At least in the last couple of games you need a highly rated player to do it.
Agree 100% that balancing this is almost mission impossible. It's easy to have them work every time. It's easy to have them either not try or fail every time. There is probably no sweet spot which will satisfy everyone.
Making them just a little too good almost ensures noobs will give up big TDs almost every game. There are also plenty of players who would be able to completely box the AI run game at the same level of effectiveness.