I think it’s a complex question that has an individual case by case definition
Just as I was turning 17, I had psychosis and my social functioning (Was rubbish anyway), was severely disrupted and it has never recovered since then
Occupationally, I was on track to go pretty far in academia, but my potential was once again beaten down by psychosis. This impairment destroyed the path I was on, and even though I tried to restart this, my 8 year hiatus means I am dreadfully out of date and behind. This is a consequence of my mental instability.
With my work as an analyst, I got to a point where I could no longer function in this role, as the family support evaporated, and I had no one giving me leeway to be off to recover from the stress and psychosis. My potential for this role could have been astronomical had I not had the mental health issues and coping strategies to deal with all this pressure and cognition to learn new skills.
Where I stand now is that I am doing a physical job, that’s low stress (For the most part), and with self employed freedoms that come with that. I feel more able to manage now.
Going back to the severe functional impairment. My level of functioning where it is now could be considered respectable by many standards imagined by a professional etc. However, I do feel robbed of the potential that I had growing up - and into early adulthood, and feel that a lot of what I could have achieved was wasted in a mass of drugs, psychosis and dealing with the aftermath of some pretty serious situations.
That for me is how my functionality has been what I would deem as severe impairment. Everything has been such a battle. Even though it feels like it has been won with relative stability for a couple of years, the damage caused is lasting, and I will probably never recover it before I die.
I will still resist, and push on.