here is a sample of 3, all broadly brought up in the same kind of environment.
kid 1, female, allergic, breastfed, to the exclusion of any solid food really until around 18 months … slept through with some light persuasion at 11 months.
kid 2, male, extremely allergic, formula fed, loved solids, no persuasion needed but in parental bed till 3, slept through by 9 months.
kid 3, female, breastfed, quite likes solids, didn’t sleep through until 6 years old.
kid 4, formula fed, high energy, ended up with diagnosis of ADHD. A bloody nightmare to get down and prone to middle of the night bursts of energy but slept through generally from 18m.
Conclusion, it is very little to do with training and lots to do with temperament.
gentle sleep training is great. I don’t think having a baby cry a bit is the worst thing in the world particularly if you’re there in the room to comfort them. Friends with multiples are very realistic about this - you can’t always attend to each infant instantly when there is more than one of them.
Ferber is a bit too weirdly Victorian and cruel for my liking….