SHORT ANSWER
Practice skin and human skin differ in stretch, ink absorption, healing, pain response, needle depth feedback, and texture. Practice skin gives you machine reps — hand speed, line control, stencil technique. It does not teach you how real skin moves, bleeds, heals, or hurts.
What transfers from pad to skin
Line control and consistency. Machine handling. Hand speed. Stencil application technique. Shading gradients (with caveats). Color packing density. Wipe technique. These are mechanical skills that benefit from repetition on any surface.
What does not transfer
Needle depth calibration (skin gives different resistance at different depths). Stretch technique (practice skin doesn't stretch). Healing response. Blood and plasma management. Client pain and anxiety management. Hygiene and cross-contamination protocols in a live environment.
The honest take
No practice surface makes you ready to tattoo people. But no one should pick up a machine for the first time on someone's skin, either. The pad is for reps. The apprenticeship is for everything else.