In this picture above, the glass isn't cracked at all so it helps the suction cup to be effective as it is hermetic. With cracked display, it won't work.
The iOpener doesn't work because the heat isn't strong enough. I used a hair dryer which proved to be much more efficient.
If you add clear packaging tape, it will create bubbles and the suction cup will become inefficient. To me it was impossible to remove the glass with the suction cup. Since the glass was very cracked, I had to resort to some tweaking with strong tape and pull the this off.