I did some research a few weeks ago and this issue has been there in one form or another for a long time. I am starting to think that this is not a bug. I moved from nexus 5x to iPhone 6s and now toiPhone 7 and both have poor quality photos. I moved back in time (on google) to see photos taken with iphone 6 and they are basically equally bad. There were some posts dating long before iPhone 6s/7 with this problem.
I somehow managed to get a better quality photos with a all settings reset (accidentally fixed it a little. I wrote about that pages ago) and my photos turn pretty ok in good light. But, yes in low light situations or with objects far away, it just looks awful.
I tested all the main apps and they all take the same photos as if they all worked on a JPG delivered by iPhone. The DNG files are better but I think that they are not sharp enough for a diigtal negative. They have more detail and some noise but are lacking shapness in my opinion.
My today's test. the darker is a JPG from lightrooms DNG and the brighter is JPG from apples stock app.
http://imgur.com/a/OQxkB
more photos comparing nexus 5x and iphone 7 in the evening. Photos taken a month ago. I never shared them because I stopped posting here.
http://imgur.com/a/Ii4EE
More saturated are from iPhone