There is a modern myth that it is always possible to get things right given the will and the training. What is not a myth, is that children need one stable carer who is always available to them at least for the first few years of life (and preferably longer) and that being moved from one to another can be disastrous for their emotional future. The highest priority should be to shift children from carer to carer as little as possible. This is a more achievable goal than the assumption that there is a formula for knowing who would be a good parent and who will not. All children need is good enough parents (and social workers) who will do their best.
Seriously unsuitable couples will not be difficult to reject. We must learn to tolerate some failure in adopters, social workers, and ourselves for it is the only way to learn. Social workers under constant fear of criticism have too little latitude to take acceptable risks, and children are the sufferers.
Bishop's Stortford, Hertfordshire