I work in welfare rights, and exclusively with clients with MH problems, but plenty of them have physically problems as well.
I do 2-4 PIP applications a month, only around 40%-50% are successful. I then appeal them. My appeals are 100% successful. (The low initial success rate isn't because I'm shite at the applications, it's a little higher than the average for my team, and the team's success rate at appeals is in the high 90's.)
If it was easy to "milk the system", I doubt if the success rate for appeals would be so high.
Staying on earnings replacement benefits because you're sick isn't easy either. It's hard to get deemed incapable of working, and the success rate for those appeals is incredibly high, too.
They're even trying to get carers back to work now. A friend is FT carer for her 15YO son, who has a autism and MH issues, and gets carer's allowance. She's also getting ESA for herself, albeit only at the basic rate (and reduced somewhat because she gets carer's allowance as well).
She was summoned to an appointment at the job centre to be interrogated about what she was doing to find a job, and her response that she was a FT carer was ignored. I'm going to go with her next time, armed with a copy of the rules and regulations.
All of which is why I know that it's hard to "milk the system" when it comes to health benefits, and that the (imo few) people that succeed should take up acting, because they must be bloody good.
If you haven't seen it, @hattie43 , I reommend the film "I, Daniel Blake". For people working in welfare rights, it felt like watching a documentary, it was so true to life.