The problem with mental health problems is it takes a supreme amount of mental effort to fill in forms and fight for your rights and there will be many people who deserve the backdated and ongoing payments but just don't have the spoons to deal with it.
That's one of the overlapping catch-22s of many disabling conditions. And of course if you can cope with filling the forms in, it can be used as evidence you're just fine.
Other pitfalls include any sort of relapsing-remitting condition, because if you're assessed on a good day, you're sunk, just as if you're seen by neighbours who don't know you well only or mostly on good days, you're sunk in terms of your reputation.
I think it's well-illustrated by bipolar disorder. Gone manic? No problem, you're just very happy or busy making up for lost time, which you must have lost by pretending to be ill. Not seen for a while, or seen out whilst low and unable to put a face on it, but not looking stereotypically depressed? Just showing signs of having realised people know you're faking it.
Ditto conditions that vary year to year or hour to hour.
