The iPhone 6 camera pickiness and inconsistent clarity and quality is well spoken about here:
https://forums.macrumors.com/threads/1790221/
In blind photo comparisons the iPhone 6 also comes in behind the S6 more often than not.
I'm not talking about blind comparisons, though. I'm discussing a specific compare with lots of pictures; and some of these pictures back-up my comments.
As for the other comments and one poster who you quoted -- could be hardware. Who knows? And more importantly why dig up one person having issues?