Definitely don’t do VO2 in ERG mode. You will have the power targets as a “guide” but ultimately your legs will guide the effort. You likely will be able to go harder on the first few and then may level out as your aerobic system gets to max uptake.
At the end of the day, it doesn’t matter.
Hear me out – in each of the situations you described, you can still rate the workout how it felt, and we’ll have the ride data to compare that to.
Again, if you lower the power to a certain point, that’s not passing the workout anymore. That applies to all workouts.
The same goes for skipping intervals or any of the other scenarios you and I described.
You rate the workout after completing it. The only time that we’ll give you a struggle survey after your initial survey is if the workout felt much harder than we think it should have.
Thanks for chiming in Eddie. If I can ask one more thing, what is that “certain point”? Or does it just vary by workout? For instance, on my 30/30 workout the other day I lowered it to 91% before the first set to see a wattage I thought I could actually do. That felt not quite hard enough so I did the rest of the workout at 92%. So just curious had I been at 97% or something, and still done all the intervals with no rest, marked it as just Hard, would I have “failed” also?
And going forward, I will not use ERG for any shorter high end intervals.
Did you get a “fail” for the one you did at 91/92%
No idea, how would I be able to tell? In my mind, it is a fail because I could not do what the workout was. But I did it at 92% with no skipping intervals, no stopping, no backpedaling.
You would get a survey asking why
(Thanks for sharing, btw. I’m struggling to understand all this, so your example is really a great test case)
Yes, I got the survey. And I answered ‘Intensity’. And like some of this stuff, I’m just hoping that is the right answer for ‘I think it was just too hard for me’. I assumed they were asking, why did you turn this down, but now I’m scared that we all interpret that differently as well.
I think it varies by the workout.
For what it’s worth, I’ve had good success with ERG mode and short intervals! 
That makes sense and fits in with what @eddie shared. Thanks again!
I think you may be overthinking 
In your 2 out of 4 intervals example…
It depends 
If I cut the workout short because of time constraints I’d answer the failure survey as ‘time” and rate the work I actually did - hard or very hard.
If I just couldn’t face anymore then I’d answer the failure survey as “intensity” and I think the system would then essentially treat the work I did as the same as it would a max effort - that’s what makes sense for me anyway.
For me a max effort answer on a passing survey is reserved for the times when I cant believe I didn’t get a failure survey. Cut short intervals, pauses in intervals, decreasing intensity just enough not to trigger a fail.
Fully agree, so yeah, maybe I am!
Update and TLDR: As always, I think this stuff is more complicated than it seems because there are so many factors.
I did this hard start workout on Thursday (The Owl +2). After all my whining about how I couldn’t do it, I was able to complete it just fine, and all on ERG mode. Think that I’ve maybe been doing too much volume lately which has made the hard workouts harder. Next week is a rest/deload week so after that I will play with the plan builder at different volumes, training approaches, etc. to see what the system thinks will actually make me stronger. But I may also need to remember that “you’re better than you think you are, you can do more than you think you can”.
It might also have been a function of getting a couple of these type workouts in your legs if you haven’t done them in a while. If I had to knock out some 30/30’s this week, I know I’d struggle with the suggested watts (even for a workout most would consider easy). I haven’t done that kind of work since last May. But after just a workout or 2, my “vo2max legs” wake up and it’s doable. For me, it’s kind of like lifting weights. If I haven’t lifted for a while, I’ll be cramping trying to squat 3x10x135 (and super sore the next day). After just a couple of those sessions, I’ll be doing sets of 3x10x185 no problem. Progress from there slows dramatically, but the initial jump is quick (same thing for vo2max for me).
That’s what I was hoping for you. They aren’t as bad as they appear.