Hmm, I think both.
If you think about it, at any given stage there are going to be children at either end of the scale. Some who are just easy, compliant, easygoing, sleep well, eat well, generally happy and you don't have to do anything to achieve this, indeed you might well assume that it's your parenting but it could just be their temperament.
Then some at the opposite end of the scale where nothing works and they are just difficult and you'll have a hard time whatever you do.
Plus a decent chunk in the middle - where they do need some active management and you can end up with a harder or easier time based on what you're putting into place. It's certainly true in general that proactive parenting is usually more effective and easier overall than reactive parenting, but it is more work upfront.
And with all of these children might be in a different category at different stages of their life. So you might get a difficult baby who turns into an angelic toddler, and then is a middle of the road preschooler, needing a bit more active management.
I was listening to a parenting expert talk the other day and she reckoned it was about 20% on either extreme, with around 60% being in the middle, which is the majority, which is probably why most parenting advice assumes that this is the universal experience.
OTOH there is a study looking at baby routine books, the ones which suggest all the standard ideas like drowsy but awake, establish a routine, get baby sleeping independently, don't feed to sleep etc, and how effective new mothers (sounds sexist, but I suppose representative of the fact it tends to be mothers doing the bulk of baby care) found the book's instructions and how it affected their overall wellbeing and confidence. This study found that the books were only effective for around 20% of the mothers in the "book group" in the study, and when they compared the group that had used the books with the group that did not use the books, they found that the book's success was correlated heavily with the mothers' senses of confidence and happiness. Basically, if the book worked for them, it increased their confidence and they were likely to recommend it to everyone, assuming that it was the book which caused the outcome. And if the book did not work, it actually reduced the mother's confidence and happiness.
Those numbers are very different because they suggest that 80% of the babies (at least in that study) are in the "nothing works" category, with only 20% total being in either "unicorn baby" or "normal baby" category.
That might just be babies, whereas the expert was talking about school aged children. It might be that the proportions of easy/medium/hard are different at different ages - there are known challenging ages, like babies, toddlers, teenagers. It doesn't really tell you whether they did the methods in the book properly. And it's only one study as well.
For sleep routines/sleep training in particular, it is difficult to find high quality research because what apparently tends to happen is that there will be families who screen themselves out because they are starting with an idea of "That definitely will not work for my baby" and/or "I am not prepared to leave my baby to cry". And sometimes people sign up for the study and then you get a fairly high rate of drop out once people are assigned to a category. So I think that actually it's probably better to look at anecdotal evidence, usually from professionals who work with large numbers of families and see a range of baby/toddler/child behaviour. But even this will be skewed because people only tend to seek professional support when things are going badly.
Anyway there is more research coming out now about temperament, which should be interesting. But my main point is that I think the OP assumes that the "difficult" babies are a very tiny percentage, and I think that it's likely to be higher than this.
Also, with baby sleep in particular I do think there is an argument FOR playing the longer game sometimes. I know that for me, I preferred to co-sleep and feed overnight and then put them in their own rooms when they were older. That worked for me and wasn't a problem, whereas a lot of baby sleep advice is presented as this kind of moral thing like this habit is "good" that is "bad" or "unhealthy" when it's nothing of the sort.