There is so much that has got to be taken into consideration now with the 6s LCD.
On the iPhone 6/5 etc you can have any grade of OCA (different widths etc) and it wouldn't really matter. Glass thickness could be different but it wouldn't matter. Now 3D touch is here everything needs to be perfect. The glass/oca/polarizer all need to be 1:1 original to maintain 3D Touch.
My opinion is don't ever use a 3rd party LCD on an iPhone 6s.