Its all about the ratio between speed and spin, not speed only or spin only.
Usually most club level players believe that if the ball comes back, it means the spin is incredible. Thats not true. A faster serve may have much more spin and may have much better spin/speed ratio.
The "ghost" serve is not difficult to learn and to make the ball come back to the net. But its difficult to master it to pass low over the net, otherwise it would be attacked severely.
A faster serve with more spin will have much lower second bounce and if the spin is enough the higher deceleration of the underspin can make it bounce twice even it may look like long or half long, and the only answer to this is a push.