I don't know if people think people are fraudsters as such more that some people including those in government think that pip shouldn't be given for conditions like depression, anxiety, adhd and to people with less severe autisim.
It's very much a question of degree. I've successfully appealed at least 2 cases where the sole diagnosis was ADHD. In both, the clients' ADHD was so severe they couldn't effectively complete anything vaguely complex, and one of them had had 2 accidental fires because they had got distracted by something while cooking. Their ADHD was so bad that their executive function was completely shot most of the time, and they had such difficulty following the thread of a conversation that they scored points for the "communicating with others" activity (although they wouldn't now, as some fairly recent case law has moved the goalposts somewhat). Iirc, they both got the lower rate of the mobility component, too, because they couldn't cross roads safely or remember where to get off the bus.
The self-neglect that can stem from significant depression often means people need prompting for real basics, so score points for preparing food, taking nutrition, washing/bathing, dressing/undressing and engaging with others.
Anxiety is a huge spectrum too. I've had a couple of clients where their anxiety is so severe they can barely string a sentence together. Working with them is incredibly challenging and time-consuming, because they are so anxious that they can only take in or relay small bits of information at a time. Doing a PIP application normally takes me around 2 hours. With clients with severe anxiety, it takes shedloads longer, and I usually have to do them over 2 or even 3 2-hour sessions, because they just fall apart unless I take things very slowly and gently.