So, i went to do a structured FTP test the other day on an indoor trainer. During parts of it you ramp up the power to exhaustion.
No matter what I do I can NOT override ERG mode and get it to stay in grade or resistance mode. When the next step comes up I goes staight back to ERG mode and I have to manually override it for that step. After which it goes straight back into ERG AGAIN. You need to maintain a constant power/pace to test FTP so this will throw the numbers off. Plus it's hard to do at high power outputs when you are breathing out your eyes.
There are people that want to do structured workouts with a range of power values, and it's literally impossible with the head unit not respecting what the user sets. Immensely frustrating.