My only smart trainer experience is with my Direto, bought in 2017. (Together with a 4iiii left only PM).
Yes, sometimes my experience as well with Power Match in Erg. Thanks for the description, because it clarifies that it’s not just the quick jaggies vibrating pretty evenly around an even center (flattened completely by Kickr’s default), nor the big spikes from shifting, standing, etc. It’s waves with a frequency of 10-30 seconds where the jaggy line consistently averages 10-30 watts above or below the target – even though cadence is “smooth”, at least within 1 or 2 rpms of the center for same period. And yes, it does interfere with the workout by increasing “cognitive load” (our new buzzword). My frontal lobe weighs the evidence and blames it on the interplay of gadgetry, as does TR support. My lizard brain if I’m tired says “Uh oh, REACT!” and I do, trying to fix it, which often compounds the problem.
Like you, I find spin down calibration of the Direto has always been consistently very close to the factory value, and enough of a PITA for various reasons that I do one only as a last resort – that has never shed any light.
And like you, I also find the left-crank PM 10-30 watts different from Direto, as repeatedly verified on @dcrainmaker 's comparison tool. However in my case the Direto reports consistently lower values than the 4iiii. Leg strength difference? Plausible, except that I know from stuff like lunges that my left leg is weaker, not stronger, which should make the 4iiii calculate lower not higher combined power.
Finally, like you, I find the more recent Power Match more usable than the old one (when I first got the Direto). That was bad enough that I just turned it off. These days the problem seems vexatiously intermittent – like the engine noise that wakes up the neighbors but disappears as soon as you get within a mile of the shop.
Another complication: I spend a lot more of my time below 200w than most people (and other Direto users) here – and remember reports saying the Direto got dicey at the low end. If so, maybe part of problem.
And two last ones: This is actually my second 4iiii left only. The first packed up after 2 1/2 good years, and Precision sent me another. It also reads about the same amount higher than the Direto! Since the 4iiii can be tweaked, a dilemma has been whether to turn it down to match the Direto, or let Power Match sort it out. Another question is whether the difference between the Direto and 4iiii is not constant, but changes at different powers – for instance, as I go from 120 watts to 220 watts. If it does, that would seem to put the whole feedback loop into some possibly weird gyrations. Is that where the “waves” come from? I don’t know. Obviously.