The problem with 3D Touch was that when it was revealed, the interactions were available on 0 phones. As a result the time it had the most visibility and marketing, the vast majority of users couldn’t try it and learn it. Since 3D Touch is a feature that is not visible in the interface at all, customer education was a key part of it being successful.
The lack of customer use and demand, as well as uncertainty about which phones it would be available on, then meant that developers often didn’t support it which in turn meant that it could become a frustrating experience for users who did use it as well.
Apple should always have simulated 3D Touch with a long press on other phones which lacked the pressure sensitivity in order to allow their customers to learn how to use 3D Touch.
Apple’s handling of 3D Touch marketing and rollout has been surprisingly poor.