Harmonic currents increase the heating in transformers and motors for the same real power loading. So you may have to derate these devices in the presence of harmonics.
Harmonic currents due to multiple non-linear loads on different phases can result in heating of the neutral.... the 3rd harmonics from each of the three phases will be in phase with each other and will therefore add in the neutral.
I'm sure there's a lot more that can be said on the subject.