"Because Power Harrows are better than Rotorvator..At everything." This is incorrect, they're better at everything, except incorporating fertilizers and power composting cover crops, the rototillers are more time efficient and effective for that task.
I've been tilling every season, basically once in spring, maybe twice on the rows where I succession crop (but just a light shallow surface till for seeding or killing weeds) and I haven't found this mythical hardpan yet. I'm sure the truth of it is that a finely pulverized soil, coupled with rain, wind and gravity compaction, and the fact that the soil in deeper layers has more weight above it, leads to harder layers of soil towards the bottom, no matter what kind of tilling you do or don't do.
If the power harrow is shredding the green manures finer than a tiller, it's quite possible the soil structure is being pulverized in the same damaging way tillers do, if the plant residue along with the soil is getting too fine we may be worse off than a tiller in that case. Less fine is ideal because you've pulverized things less, once you break it up by mowing it and get it down in the soil nature will do the rest to make it availalble to future plants. Rototillers and harrows are quite similar, one blade turns vertically and one turns horizontally. Horizontally, soil layers are better preserved, which is good a lot of the time but not when you want to work in fertilizers. If you want to incorporate fertilizers such as chicken manure or green manures, by definition you don't want to preserve the soil structure, because you are trying to blend the fertilizer into the soil evenly throughout the soil, and you want to swiftly move it down into the rootzone of the future plants, thus mixing and changing soil structure. It disrupts the soil ecosystem (bad) but adds needed fertilizers for the new plants (good). The soil ecosystems will re-establish themselves. Hopefully the good outweighs the bad, if not we'd be better to use the Back to Eden "forest floor" method of fertilizing where everything starts as roughage and wood chips on the top, slowly breaks down, eventually forming a nice soil without disrupting anything below, but it takes a lot of time! When we're market gardening for profit with succession cropping we simply don't have that time.
Could a power harrow ultimately do an equal job as a tiller in incorporating fertilizer? MAYBE. If you ran at high enough rpms, made several passes, burned more gas, as I said we aren't physically working the fertilizers down with the power harrow, we are simply displacing soil and letting gravity do its work. On top of that the power harrow doesn't go as deep, tillers are 8" or better for my Grillo tiller, power harrows less, well roots go a lot deeper than even 8" but deeper is better in the case of mixing fertilizers.
In summary, no, you can't have your cake and eat it, too. The power harrow can't both be "better and gentler on soil because it preserves soil structure and doesn't invert or blend the soil layers" and at the same time "better than a tiller at evenly mixing fertilizer from the top of the soil down into the bottom soil layers." If the power harrow worked perfectly for the aim of not inverting soil or mixing soil layers, it wouldn't work for mixing fertilizer at all. It's only because it does in fact mix soil and layers to some extent that it works for incorporating fertilizers at all. When your device is working at one task only because of it's design shortcomings (the unwanted mixing of soil layers), then you know for sure you do NOT have the most efficient tool for the task at hand.
I wonder if anyone can chime in on disc harrows vs power harrows vs rototillers for incorporating crop residue, green manures and fertilizers into the soil. I'm pretty impressed with disc harrows myself, such simple machines that achieve a result very close to the pto powered tillage implements, and much faster/less gas burned.
