I posted about this a year or so on the NSM Strava group. I went back to NSM vanilla. You are getting quite a lot of load/stimulus from training at a much higher easier HR for the easy running in this and ultimately that's what got me and burned me out. There's only so many ways you can balance things I guess and no method is going to be for everyone.
I think this method will be pretty good if you follow it and your Lt1 is actually quite high, you probably can get away with higher HR efforts/ paces on what for me are really overly aggressive easuly running.
Just my view, but going back to much easier running, then the only way to get enough stimulus in was to go back to 3 workouts a week at Subthreshold. Then I started to feel much less burnt out again, absorbing a lot more easy miles.
I had more fun doing this, but the results and sustainability just weren't there for me. This actually more in like with what KI was doing, just putting the work into a smaller amount of time rather than his 3 sessions. Certainly in how aggressive it is with easy running, as he did and there's been lots of debate on if he simly is cooking himself long term on easy days. Although, I do see an argument for especially on really limited hours getting a bit more aggressive with easy paces.
Glad to see some people are having some success with it! But I do think this definitely requires a pretty specific type of phisiology that is inherently linked to easy days.