From watching the game on TV it looked to me that there were a few big intentional grounding calls missed on Rivers. One in particular where the ref said the qb was outside the tackles when he clearly wasn't.
Was that challengable? I feel I've seen that challenged in the past.
One question does come to mind: if it were ruled a safety, then it would be a scoring play. So could a review be done? I don't think so.
In the rule book, it says that there is a review when there is an on-field ruling of a score. In this case there was no such ruling.