Here's another way of thinking about it. In an Active Front End, harmonics are CREATED that cancel out the other harmonics, but they are not created from thin air. There is no net gain in energy efficiency by using an AFE for the purpose of harmonic mitigation. There is a SLIGHT improvement in overall throughput efficiency by virtue of lower losses in the distribution system feeding the VFD, but it is not in and of itself a reason to use it.
Some mfrs taut energy savings from using AFE drives, but that is for when there is a lot of regenerative energy available in the system where the AFE can be used to pump that regen energy back to the supply, which does have the effect of lowering your energy bills. Then in another sentence on the brochure the AFE suppliers talk about how an AFE drive can mitigate harmonics. What they DON'T tell you up front is that those two feature are essentially mutually exclusive, they go ahead and allow you to infer that you get both. You are EITHER going to recapture some energy when slowing down a load, OR you are going to mitigate harmonics, but not at the same time.