Well, when they were first introduced, you had extremely low alk, low Mg, high nitrates, and slightly high phosphates. That's a lot of parameters not in ideal levels to be introduced to during a stressful event as a tank transfer itself.
I'm not suprised the birdsnest died. For a good beginner coral, they are just sometimes finicky and will die quickly if not happy. I've seen a colony melt over night when not happy... something I rarely see with acros, as it's more of a slow STN event. I've only had small frags of acros RTN overnight on me, but never a full colony like a birdsnest will do. My guess is they are hardier in the sense of unfavorable water conditions, but more prone to bacterial infections that can wipe a colony.
Either case, your frags were probably stressed after introduction to your tank and then during the changes that were occurring to get the parameters in the correct ranges. Most SPS will typically respond to stressor events a week or so after the actual event. It's kind of a delayed negative response sometimes. You'll understand what I'm talking about once you've had old colonies sitting around and then you have an alk spike or drop and nothing happens to your SPS. Then 1-2 weeks later you start getting STN.
The remaining montis and SPS you picked up are good tester corals and are super hardy so I'm sure they'll recover.