There clearly was a discoverability issue with 3D Touch. When you knew which app supported the feature, it was awesome but there was no way of knowing in advance if there was support for it before using it.
Long pressing an icon or anything just to see that nothing happens was just frustrating. Long term effect was that users probably felt this wasn't reliable/supported enough to use the feature at all or at least not in the scale Apple wanted.
edit : fixed typos