I have no issue with the Cummins front foot call. I saw that one live, and it was Umpire Dharmasena who was telling the producer to go back a couple of frames in order to find the first point of contact. Looking at the earlier frames was actually an attempt to help Cummins, as an earlier contact of a moving foot would have been further behind the line. Correct direction by the umpire, correct execution by the technician, and the correct decision made in the end.
My verdict: Australia got away with bad cricket, if he hadn't been going so close there wouldn't have been any risk of it going the wrong way.

The Cummins caught behind one was a tight one, but the correct procedure was followed and the correct call made according to the regulations. There wasn't enough to show that the umpire's decision was wrong, which is how the system works.
Verdict: tough edge case that is a predictable consequence of the system working on 'umpire is assumed to be right' rather than 'be sure it is out' - I think it was the 'white' faction of ICC full members (England, Australia, NZ, South Africa) which insisted on this.

I didn't see the Bairstow one, but based on how I had seen the protocol followed properly every other time I can only assume it was correct. Maybe bringing back Hot Spot (dropped due to petty disputes over who would pay for it - ICC should have bought 3 complete sets of the equipment) would have helped with that one?

Personally, I think it's time for a couple more modifications to be made to the DRS:
1. No balls to be checked on replays all the time (not just wickets) so bowlers can adjust their run up before having a wicket overturned.
2. Fielding side to be penalised 5 runs for an incorrect referral.
3. Time wasted during incorrect referrals to be added to the referring team's time for the over rate calculation.
4. 'Umpire's call' to include slightly missing the stumps as well as slightly hitting.