I think ProfessorLayton about sums it up.
I hold my hand up and admit that I am guilty of occasionally being brutally honest about my experience. And I do it for two reasons. Firstly, I'm trying to help. I'm never saying this will happen, or it will be this way for you but I am saying if it does, if you do find yourself feeling this way or experiencing these things, you are not doing it wrong, you are not failing, you are not a bad mother. I closed down in pregnancy to the horror stories, to any story of instrumental deliveries or cs or panic or worry because I didn't want to hear it. And dear God I wish I'd listened. I understand now what my friends were doing. The gap between expectation (my expectations, my family's expectations, the expectations of a sleb culture where you can do it all and bounce back from birth in minutes) and the reality was the gap I fell into. And I ended up with PTSD that robbed me of months of my dd's life. Because I didn't want to hear it. It was my mistake. I am now one of those women who is probably painted as an interfering old nag. And I don't tell horror stories or scare people but I do try and reassure that there's a wealth of normal out there and you're not alone.
Secondly, you'll find that people need to share their experiences. Especially when they've had a difficult time, particularly the tragedy of losing a baby. Them talking about it is their need to deal with it. It's difficult and might seem insensitive but they are dealing with their own grief, not wishing it on you.
And the good bits. The sheer, overwhelming, beautiful, wondrous experience of creating a life and having the privilege of raising a child is not something you can put into words. Chilbirth fucking hurt is quite emphatic and straightforward, the mystery of holding your own child for the first time is not something mere words can express. That's why we try and sum it up as 'you have no idea' because really nothing's comparable.
I try to approach it from the point of view of largely, people are trying to help. Equally, people are sharing a sort of dark humour with you. Because new babies are shit and vomit and lack of sleep and the only way through it sometimes is to try and share it with other people and laugh. At the same time, yes there are idiots out there who say stupid things but that's nothing to do with having a baby, they're just hard of thinking.