Another long one, but I find these discussions engaging (& hopefully helpful for others who are just lurking the forum) 
This is a great question! What makes a workout or series of workouts “best”, or “better” than others?
And would your answer apply universally for all athletes wanting to build their 5 min power? If so, shouldn’t everyone follow the exact same set of optimal workouts to build their 5 min power in the most optimal/efficient way?
We would say that as long as the athlete achieved the strain needed to achieve progressive overload, then yes, they had a great session, whether they did them in an “organized” manner or not. You could accumulate the strain by doing a structured 5x5 VO2max workout (which is very efficient, but also very challenging), or by spreading out those efforts over a longer ride. They might require different total amounts of work, but generally yes, we would say they’re equivalent if the strain is equivalent.
I do want to emphasize that Buckets are not equal to Time in Zone, since XSS accounts for the effect of fatigue. For example, if you were to do 5x5 VO2max intervals, Xert will give you more “credit” for the last minute of each interval compared to the first minute because the latter is performed with significant fatigue. The same is not true of other training scores based on time in zones or average power - since they have no ability to account for fatigue as the intervals are done. To them, each minute of each interval is equivalent since they’re at the same power output.
In my (biased) opinion, our approach is actually quite simple: Do more (Low/High/Peak) XSS and your fitness (TP/HIE/PP, respectively) will improve! The smaller details might not materially matter (e.g. high intensity 2x’s per week vs. 3x’s per week, Micro versus long intervals, etc.) as long as you’re achieving progressive overload.
There isn’t really much of a black box or stressing about picking the “best” workouts. Do structured workouts if you’d like and you think they help! Or, don’t do them if you don’t like doing structured workouts.. It’s not the structure of intervals that make you faster, but actually the work of doing those workouts that makes you fitter!
The only “magic” that we do is calculate the target power that will apply XSS in equal proportions to the systems you need to train, which is what Magic Buckets does interactively as you ride
This new features is helping many athletes get the training that they need done, one way or another! If you’re skeptical, you could even combine a standard/traditional workout with Magic Buckets… do your normal workout first, then “top up” with Magic Buckets at the end. I found this a fun way to train outdoors a few times last year:
Lastly, anecdotal evidence for now (along with reported success from others), but I have had great success with my winter training so far. I’ve only done 2 structured “workouts” - the rest of my intensity so far has come from a combination of Magic Buckets & Zwift racing. I’ve already increased my Threshold by ~8% with more room to continue growing (I’m currently only at 3-star fitness- hoping to reach my 4th star in time for outdoor season):
Thanks! I’m starting my participating in the study later this week - I’m excited to contribute my data! I’m hoping it will help motivate me to finally reach my past levels of peak fitness, if possible.
If others are keen to participate, there’s still time! Details here: Participants Needed!