It would be interesting to see the stats for open adoption in the USA where the child has been removed for child protection reasons, rather than relinquished because of poverty or stigma.
To generalise completely (and I agree that every case should be looked at individually), birth parents who relinquish because they don't feel able to parent in their current circumstances are more likely to be able to maintain a stable relationship with agreed boundaries with the adoptive family.
We had an open adoption, and while it has been positive overall (I think), it certainly caught up DDs in a degree of chaos that wasn't always good for them. Not because birth family didn't care, or were malicious, but simply because their lives were so chaotic. It was like living in a soap opera at times.
I was DDs respite carer, then foster parent, then adopted them, so our case was not the typical situation. It was the best solution for us. But since their birth parents died DDs have drifted away from extended family, and kept the relationships that provide nurture (previous carers) but not the disruption.
As others have said, long term foster care is not a good solution either. Back in August the Today programme on Radio 4 focussed on foster care with Lemm Sissay, who interviewed a trans-gender child with multiple foster placements. When asked what they think should change about foster care, to make it better for the children involved, they said that foster parents should not be able to end a placement easily, that they should be more committed. I listened to that thinking 'hmmmm, sounds like advocating adoption to me, the point about foster care is that the carers are NOT parents who have to stick it out'.