I have been doing a few bikes lately. Its much different to polishing car paint as the surface under the paint is usually different.
"Why does it make a difference, paint's paint, just give it a quick polish?"
It's more complicated than that because the because the polisher, pads and polishs will react differently to each of the different surfaces because of the way they dissipate heat. Metal surfaces will disperse that heat especially doing Aluminium bonnets whereas some surfaces such as plastic, fibreglass, carbon fibre will not. For this reason it can become dangerous if these substrates are not identified and taken care of separately.