It should be based on the functional impact of a severe and prolonged impairment, rather than just a medical diagnosis.
It is based on that, really; after all, a diagnosis of cancer can mean anything from a devastating terminal illness to a small skin lesion that can be removed in minutes.
But it is very worrying. Quite a number of people who hadn't been invited to migrate from ESA to Universal Credit by late last year decided to start the process themselves, since if they managed it before the cutoff date, they wouldn't be affected by the rule change. But that doesn't help new claimants, who could be getting less support than some existing ones even though they have greater impairment.
One thing Britain's good at, though, is welfare advocacy. It's first class and available at very low cost. Advocates always find at least some opportunities within the law, and helpful case law also develops via decisions handed down by judges.
Many of the welfare changes that governments have brought in over the last 30 years reflect this. Each set of changes is aimed at reducing the number of people who qualify, but the number tends to increase over time as people find ways to make the law work for them—and because their impairments are genuine and substantial, tribunals often find in their favour. Judges sometimes look more closely than DWP assessors, and also consider what is a reasonable interpretation of the law.
It's a kind of elaborate dance, but even though claimants' chances often improve, a
lot of sick and disabled people get trampled underfoot along the way. And of course you have a better chance if you're well educated, not mentally fragile, and have people who can offer you practical support.