My oldest two could not do sleep training. 10 months of trying for almost every night and they did not just adapt. My middle daughter embraced it after a single night. My adopted middle son didn’t need it at all and was naturally “good” at sleeping. My youngest daughter and son will fall asleep right away if around someone, but otherwise will stay up for hours (but never cried about it).
Again, one family experience but each child has their own needs and responses. I’ve never found a single method that works universally in any aspect of parenting.
Except ice cream. They all seem to love that.