Any after market device/trigger shoe one puts on, has the risk of failing (or working) at the wrong time. Since the most common method of mounting is to clamp onto the original trigger with set screws (or any similar method), it effective widens the trigger and increase the possibility of an UD. Any add on part other than a manufactured trigger designed with the increased pad depth area built in is asking for a problem.
Most of the UDs I investigated involved handguns and rifles with trigger shoes happened when the firearm was being holstered, handled (loaded) in a manner that resulted in the trigger being touched inadvertantly, or inserted in a case where the increased width caused the trigger to be pushed back. Yes, I know, the idiots shouldn't have been putting loaded firearms into cases (even with the safety on), but the handguns were being inserted into holsters not designed for the shoe.
Also, any increased front area of the trigger decreases the space available for ones trigger finger in the front of the trigger guard and that could result in an UD by simply inserting the finger in the trigger guard.