We've had stick from the grandparents about co-sleeping, rocking / feeding to sleep etc.
Our parents didn't have access to the scientific research we have now. We now know that there are lots of benefits to babies being cuddled, helped to sleep, and we know about some of the potential health implications of too early 'sleep training'.
My mum was told of instances of people rolling on their babies in the night so she gets scared at us co-sleeping, but research now shows it's safe if you're bf and not using drugs/alcohol/smoking etc.
I spent ages in the early days trying to get DS to 'self-settle'. It was a real waste of time and next time I'd not even bother.
Until 14 weeks he would only sleep in the sling in the day. Then rocking worked, then feeding, then he briefly self-settled, back to feeding, then at 8 months that stopped working in the day and now I rock him again. The sling doesn't work any more, but the pram does even though it didn't use to.
It changes all the time and I think they do get it - anecdotes I've heard vary from 6 weeks to 4 years.
The No-Cry Sleep Solution is a good read for babies from 4 months up. And after about 3 months it gets easier. Before then babies go straight into REM sleep, which means they're twitchy and wake very easily. After around 3 months, they go straight into non-REM sleep which means they're much less likely to wake when you put them down.