You could also ask this question –
Are there cases where the RAMP or 20-minute test model doesn’t work? Answer: YUP!
“All models are wrong but some are useful.” 
FTP is not your 1-hour power (the myth that will never die) but for some it’s close. 
All population stats are subject to a bell curve. Example –
^^^^^ THIS ^^^^^
What is it about your Xert derived signature that prevents you from effectively training?
Are ALL HIT workouts too hard under ERG control?
Is it an issue when workouts are converted to %FTP intervals and pushed to Zwift?
Or are you primarily concerned with successfully executing VO2max intervals?
VO2max is a physiological state you want to reach at whatever duration you’re able to hold (2-5 minutes) using whatever protocol it takes to get you there. That could be traditional straight blocks at a %TP that works for you or micro-interval sets (Ronnestad-style) or hard-start intervals (ex. Closer series). Choose whichever method works best for you at whatever intensity is required. Ride in Slope mode if necessary. The goal of VO2max training is not workout compliance under ERG control. It’s get there and hang there to the best of your ability on that day.
While I was originally enthralled that I could complete most HIT workouts at 100% compliance in AUTO mode (when status stars equal diamond count), I have since graduated to a foolproof never-fail-a-workout method. It’s called Slope mode + XMB to fill any remaining buckets.
Extract can be run when the activity includes a BT or sufficient maximal efforts (MPA close to power). Otherwise use Refresh/Save to tweak a signature.
Extract versus seed a signature –