I know many people, young and old, with longterm health conditions including severe mental/physical impairments or significant learning disabilities who have a very good quality of life - they may wish to have to deal with whatever health condition they are stuck with but their life is one that includes enjoyment and purpose, not just suffering.
I also know people whose life from the outside in looks great but they are unable to enjoy what they've got and they themselves would say their quality of life is poor.
I think QoL is mainly a subjective measure and whenever at all possible the person affected by whatever problems MUST be part of the conversation.
Obviously, in the case of a baby or demented adult or unconscious person that conversation is forced on to third parties which is so much more difficult.
Standardised scores for things that are not objectively measurable (pain, distress, QoL etc) have their place and are certainly valuable tools, but I worry about them becoming set in stone, rather than just a guide.
Communication, communication, communication IMO is more important, generally.