Fwiw, a few observations suggested by my own experience, using HIIT as one of several training tools:
It's not something to be done casually or as the way to start getting in shape. If a friend wanted to start doing HIIT, I'd urge a check-up and doctor's advice first.
It really is "brutally unpleasant," but for some reason I find it much less aversive than longer intervals at lower intensity. I've wondered whether it may be because the worst feeling (at least for me) comes after a given interval is over, early in the rest phase. Anyone else observed this?
A good training foundation before starting HIIT, warm-up before doing HIIT and cool-down afterwards, adequate recovery time and rest in between workouts, and avoiding spending too many days/weeks in a cycle of HIIT are all important.
"If you want to hike far and fast, you have to train doing both."
Well, it depends on what you mean by "far and fast," but it's quite possible to prep well for continuous mountainous 20-25 mile days with twice-weekly hikes of ~ 5-6 miles plus a robust aerobic conditioning program that can be based in part on HIIT. It's a useful approach, since most people can't spend the time for 20-25 mile days during the training phase.