A digital servo develops its max power just 2 degrees off center, so if you have the slightest setup issue and overdrive your flap such that it hits the stop while being told to move, the servo can do one of two things;

1) Draw stalled current putting you at risk of running your batteries very quickly.

2) Burning out the servo motor and ultimately giving you an asymetric flap extension..

Both of these would be very bad.

Because an analog servo doesn't develop max torque until around 17 degrees off center, this becomes much less of an issue. Also, I don't like running digitals on anything that will experience a continuous high power demand like a flap. A regular control surface needs a lot of power but only for momentary bursts.. whereas a flap needs it the entire time it's deflected and ultimately can put you into the same situation as above ESPECIALLY if you put the flaps down at a higher speed than recommended.