I noticed things were failing a bit more than they should be. It was ridiculous, On several occasions I tried 16-26-36-46-56% and all but one rush failed.
I think the Luck stat of your dwellers has a hidden effect on the chances of rush failing.
I ran 16% 60 times and it failed 8 times.
I ran 26% 75 times and it failed 20 times.
I ran 36% 60 times and it failed 30 times.
I ran 46% 40 times and it failed 21 times.
I ran 56% 40 times and it failed 28 times.
I ran 66% 45 times and it failed 37 times.
I ran 76% 50 times and it failed 45 times.
I ran 86% 48 times and it failed 44 times.
So something is a little off somewhere. Obviously a greater sample size would be more accurate but I don't have time. But I think there's more to rush mode than it seems.