Quoted from RWH:
The worst thing about the new rule is so far it has just made things worse. They go back and look and still get the call wrong! I think it will go away next season or I'm hoping it does,
The challenge is only for pass interference. So in one of the games they were trying to see if the contact was before the ball left the qbs arm, (holding) or after (PI). Since the blatant foul happened just before the ball was thrown it was not reviewable. And yes, other fouls are discovered while reviewing for PI. Which makes the review so arbitrary. Why review for PI and not holding, illegal picks, etc? It just pisses everyone off and delays the game more.