In my opinion, the metric that matters most is the enumeration of necessities that a mentally ill person cannot have due to their illness.
If someone's illness makes them homeless, that's important.
If someone's illness makes it impossible to hold down a job, that's important.
If someone's illness keeps them from tending to their health and hygiene, that's important.
If someone's illness prevents them from eating or sleeping, that's important.
If someone's illness prevents them from forming viable relationships with other humans, that's important.
... etc.
These kinds of dysfunctions, taken together, make dealing with all of them harder, too. They do not compartmentalize well.
This makes the size of the set of those dysfunctions the key metric in assessing the severity of mental illness.