If you use CC in a sensible and loving way, it is perfectly fine for a baby of 6 months plus, in my opinion. Of course I'm only basing this on my own son, so hardly a scientific point of view.
I started it at 6 months on the dot, for the odd nights that he wouldn't go down on his own. He was pretty much sleeping through by then (I mean 7-8ish to 5am ish, followed by a couple of hours in with me) so it was only initial bedtime. My thoughts -
Make sure the child is fed, watered, clean and tired. If they get distressed to the point of screaming then stop, cuddle, soothe and try again when they are calm. If it goes on for more than 30 minutes, stop, cuddle, soothe and try again. I would never have left DS if he was screaming, only whinging/tired crying. I would not have kept it up for 3 hours. I was not averse to co-sleeping and would do that if DS really wasn't playing ball.
For us, it wasn't a magic, 3 nights and he's fixed, solution. He would self settle most nights, and on the nights he didn't, we did CC. Those nights got fewer and further between until he was pretty much 100% ok self settling.
Once again, not a scientific study, just my own son - who is probably a naturally good sleeper anyway - given his father's sleep addiction issue but I do understand the issues around stress, attachment and brain development and my son is a very well adjusted, happy, healthily attached child of 3 who has no fear of bedtime and sleeps brilliantly. I believe that there is a world of difference between attempting to force a child who is not ready to self settle, and assisting a child who is very ready. CC should only be used for the second group and if your DD isn't ready for that at 6mo then she isn't. If you are at it for up to 3 hours then I'd venture to say she's not quite ready - but do bear it in mind for a few months down the line. I'd advise you to continue with co-sleeping when she wakes up at night for a little while longer.