In each case, you have a particular wavelength of light being emitted. When using an IPL, because you are taking, for example 15% of the total waveform, the energy being absorbed by the client is far lower than that which could be emitted by a laser. Furthermore, with IPL’s the filters are often not perfect functions, and allow a larger variety of wavelengths through than that which is emitted by a laser system. This is not necessarily a bad thing, as when using IPL for hair removal there are wavelengths produced that are closer to that of skin rejuvenation services, thus having the side benefit of improving skin quality and helping with any skin irregularities. On the flipside however, this skin rejuvenation effect can be problematic when treating conditions such as port wine stain or complex vascular/pigmentation issues, because the excess heat produced to the surrounding tissue can increase the risk of adverse effects for the client.
This reduction in power from only taking a small percentage of the waveform is the single largest difference, but not for the reasons many think.