The depth of penetration of intense pulsed light (IPL) varies with the cutoff filter used. The IPL light is broadband light and the handpiece used for a particular application determines the more specific wavelengths emitted. That being said, IPL wavelengths don't penetrate into the dermis because of reflection and refraction of the light and the shorter wavelengths used.
The nonspecific bulk heating associated with IPL devices is the real issue because targeting chromophores is more difficult with broadband light. Either way, in good hands, IPL is certainly safe on the neck and is generally well tolerated.