If there is one gimmick, it is live photos. I thought touch 3D required the hardware of the 6s/6splus? Interesting.
I get that from a 6plus to a 6splus a $800 upgrade is probably not worth it. BUT I needed to get my wife out of the 5s, and into a 6, and thus for me the apple upgrade program for the 6s was well worth it. In a year, I'll have a 7. But I do think the 6s is lightning fast, and I have been very pleased with it.
But the article you cite on the camera refers to problems in low light. NO camera with a sensor as small as on a smart phone is going to do extremely well in low light. BUT in outdoor photography, the 6s really soars. Is it revolutionary? No. Is it a big improvement, yes. As far as 4k video, I have never turned it on.
My point is that for anyone unhappy with the upgrade, there are others who are pleased.