It took me maybe a week of use to understand it. Following that, never failed me even a single time.
In what way was it unreliable for you?
I could never get the 3D Touch switching working reliably on my regular iPhone 7 with silicone bumper case.
I'm guessing it's because the bumper case adds a slightly raised lip around the edges making it impossible to touch the edge of the phone's screen. Never had any problems with the back swipe gesture, but 3d touch switching was near impossible for me to activate.