The problem with HIIT, is it’s not clearly defined what it is.
IIRC, it became popular after being on a documentary where the sample athlete showed good improvement from 3×30 second maximal sprints on an exercise bike 3 times a week.
But this seems to have spawned any number of different variations calling themselves HIIT, with no scientific evidence behind most of them.
Personally, I think it sort of fits in right along with fad diets – probably an OK shortcut for a few weeks, but not a replacement for regular longer exercise sessions long term