Well, other than say a 20 min FTP test? This article suggests that Erg mode might not be great for short intervals. I realise why, but I have never tried any workout without Erg mode.
For ERG mode to properly kick in, you need to give it around 30 seconds. That’s usually the time it take most trainers to adjust and stabilize resistance. So I usually avoid ERG mode with intervals that are less than 1-minute in length. Usually Vo2 max or sprints, I like to disable ERG mode.