Any benefit is going to have a few people who are clever at playing the system, but I don't think it's as easy as people think it is with PIP.
My SD, 17, has AuDHD and claims to have some physical issues which can't be diagnosed (I think she has health anxiety which her Mum has encouraged in her since being very young). She's doing well at school, works two jobs, has tons of hobbies, her own dog, and an active social life, volunteers and has done silver D of E.
When she turned 16, her Mum decided SD was going to apply for PIP. SD was adamant that AuDHD would "qualify" her for it and pointed to things like having a reminder on her phone to take her meds. She said she knew loads of people just like her who got it.
I felt like you, I thought it was outrageous that someone like her would get "free money". She openly said she planned to spend it on days out because she gets bored more than non-ADHD people!
I pre-judged the situation based on what she THOUGHT she could get. However, she got zero points.
So I think there's a lot of people talking about who CAN get it, which leads to a lot of people applying and a lot of people being quite annoyed about that, but it's not actually reflective of who DOES get it.