imo placing at nats is somewhat easier than some invys as multiple states can have amazing tester/builder in a single event, but due to the team format, they never actually make it to nats, sometimes even states. An example of this would be dxu's case, where their team didn't even make nats, while the team that did was still able to come close to medaling. Also lowkey some states, have teams that are good for the state, but fall easily when it comes to nats
At what invites would it be harder to place at then nats? Obviously some very good people don't get to go to nats, but no invite has that many high quality teams as nats. MIT and SOUP are probably the hardest invites this year, MIT was missing teams like Troy, Solon, Mason etc, SOUP was missing teams that went to Cornell, and both were missing lots of teams that just didn't go (Mounds View and 7 Lakes just to name a couple).
Also lots of things- people can't come, conflicts, poor tests, style of tests, really putting in extra effort for nats, having a good/bad day, or even just lucky guessing can change results significantly.