The biggest problem with ATOS is, they are paid by results. So in effect they will fail people simply because the more they fail, the more they are paid by the government.
My partner was on DLA and went for an assessment with ATOS. What she had was not and couldn't be called anything like a medical assessment. They asked her no questions about her medical condition whatsoever, did no medical tests at all.
Simple questions like "Can you make yourself a cup of tea if your partner's out at work?" "Can you dress yourself?" "Can you sit for at least an hour without discomfort?" "Can you walk a hundred yards without stopping?"
I was at the assessment with her and they didn't once ask me anything about her or her needs.
A few weeks later she received a letter through saying she had failed to meet any requirements to continue receiving DLA.
I thought the whole assessment they did was a farce. The only person there with anything approaching a medical qualification was their appointed first aider. Her own doctor, has written to ATOS to try and get a copy of the assessment they did (so far with no response from them).
PS apparently, so far ATOS have failed around 95% of people referred to them by the benefits department.