Not convinced
That 17% claimed no burnout....
Agile or non agile methodologies, problem always is manglement wanting the outcome in a timescale that is not feasible without corner cutting (e.g. poor code quality / bugs, lack of tests, some functionality problematic (e.g. edge cases) or even missing etc. ) and the general continual treadmill of no chance to take a breather, improve your skills (lots of employers make nice noises about training, work life balance etc., but reality so often tends to be that any (significant, as opposed to a few token hours here and there) upskilling you want to do is in your own time as workload allowances not adjusted accordingly).
Maybe I'm unlucky, but I have worked for companies ranging from small to huge, and as perm & contractor over the years, but always the case of someone high up agreeing to an unrealistic deadline & those at the coalface suffering the fallout.
As others have said, at UK pay rates, its a lot of stress for a poor salary (& plenty of studies to show that a better salary makes people better able to deal with high stress because at least that removes the general "struggling to get by financially" elements of general life stress & so only leaves you with job related stress & non economic based general life stresses).