To be fair, in my experience the vast majority of the prepared who've researched and are determined do make it through the adoption process
I've talked at two prep groups and talked quite a bit to the social workers who handle initial information calls (and info evenings etc). My council do turn a lot of people down from the first phone call, but these arent people you would want to adopt or they can't legally/practically adopt at this point in time - people who start by asking how much you get paid to adopt, they make it rudely clear that they only want a newborn baby with no problematic background issues at all, they are under 21, they want an older child but they have 2 small birth children, or they are currently undergoing fertility treatment and want to adopt whilst having IVF. Etc - that's all genuinely true. Those people will all either get turned down flat or advised that they can apply in future once xyz has happened
If you've actually researched the requirements and policies and what adoption entails and who the children are in the 21st century...then you are highly likely to get invited to an evening and to have an initial visit. More people drop out then, not because SS told them they couldn't but because they realised that they aren't ready to adopt yet - eg. they realise they need one more round of fertility treatment before they are emotionally ready to move on.
I know adopters who have had significant mental health issues in the past, who have conditions like cystic fibrosis, who are single or gay like me, who have past criminal records, who are middle class, working class, white, black, asian. All of them made it through because they were prepared and stable and ready to discuss it all through
Certainly some people are turned down when they shouldn't be, but at least where I live they really aren't finding excuses to turn people down. Can't speak for other councils or agencies
Also the process to approval is now supposed to be concluded in 6 months and most agencies have now implemented it from July 2013, so it's not too long now either. The wait will come when you are seeking a good match