There’s no reason that the measurements that go into a quality-of-life index are necessarily more subjective than those that feed into GDP. It’d be a very good idea to use measures that are hard numbers (life expectancy, infant mortality, days lost from work due to illness, levels of pollutants, etc), and even better, ones that are not easily gamed (say, NHS queue lengths, which have been manipulated in the past). But that’s an issue with any choice of metrics.
The relative weightings, on the other hand, depend on political priorities and values, so there will be some subjectivity in those.
There’s no reason that the measurements that go into a quality-of-life index are necessarily more subjective than those that feed into GDP. It’d be a very good idea to use measures that are hard numbers (life expectancy, infant mortality, days lost from work due to illness, levels of pollutants, etc), and even better, ones that are not easily gamed (say, NHS queue lengths, which have been manipulated in the past). But that’s an issue with any choice of metrics.
The relative weightings, on the other hand, depend on political priorities and values, so there will be some subjectivity in those.