I think the biggest problem with any disability claim is that it is so difficult for people to be totally objective about their own limitations. I hear so many say 'oh yes I can do that...' but when we dig a bit deeper they can't do X without adapting and making allowances.
That is why so many claims fail, and so many people who should get - for example DLA - don't get it. And that is also why having help from an advisor can make the difference. It can be a tough thing to face, and upsetting for many claimants, but the end results can be worth it.