Happiness is a purely subjective thing. It's plainly obvious that any attempt at such comparisons will be doomed to be of limited utility. There are plenty of other ways you could try to go about getting something more useful, but none of them will be perfect.

The good news is that we don't need a perfect happiness report to think about the things various countries are either doing very well or very poorly and how our own lives might be changed if the place where we live did things differently. The World Happiness Reports gets attention year after year because it prompts that kind of thinking and there is value in that.