I see lots of workouts that are supposedly done in ERG mode, but have intervals of 30s,20s, 15s and even shorter. Are people really doing these in ERG, or are they doing them in slope mode? I’m not talking about Breakthrough workouts. See Say it Ain’t So 60. There are some 15s intervals in there. I expect any trainer to take at least 4 seconds to power up, so 15 seconds become more like 10 seconds.
Yeap, I go with the slope mode. Even for the other workouts, with no short intensity interval, I prefer to adjust my power and cadence “by myself”, not by letting the ERG mode decide for me. And on my HT, the ERG mode always fluctuates the resistance by “too much”, sometimes 20W below, sometimes 20W above, that can be very hard when you want to maintain your cadence and when you are already hitting high power. The downside is that you need to check your smartphone all the time, to be sure that you are in the correct zone.
I guess it depends on your trainer, but wouldn’t the delay affect both start and end?
I don’t have any issues riding short intervals in AUTO mode.
Here’s an example from Say It Ain’t So - 60:
Some trainer apps let you adjust the transition sensitivity.
If a big jump is next, I rely on the Cadence Optimizer gauge to rev up a couple seconds before it hits.
Note: For any newbies who stumble onto this thread, ERG mode is AUTO in Xert. 
What Trainer Modes Are Available in Xert’s Workout Players? – Xert (baronbiosys.com)
I use erg, but only because I have a retired mtb on the trainer with a 28T on the front and normal 11-42 cassette on the rear. Without erg I can meet the power targets initially but they then tail off unless I pedal like a madman and even then it is difficult to maintain over 310 watts. When the drivetrain needs replacing I’ll remedy that 
I find my trainer reacts quite well, does take 2 or so seconds to get to correct wattage but when the effort ends it also takes a couple of seconds to revert back so is not to bad in reality.
Here’s another example comparing Tabata’s (20 sec ON, 10 sec OFF) in AUTO mode –
I certainly got the intended strain at defined durations on this set, but there were five sets of the darn things for this workout. I fell apart by set #3 and finished in Slope mode so I could at least muddle through the remainder. Not ready for Tabata 10s RIBs. Perhaps when I get to 4 stars which is months away at current IR. This workout wasn’t recommended but I thought I’d try something different while form was blue.
My suggestion for EBC would be to modify the mode switcher to support toggling AUTO with Slope. You wouldn’t have to cycle through all five settings (AUTO, Slope, ERG, Resistance, Off) and could quickly switch if you run into trouble or your trainer transition response time isn’t up to the task. Slope mode provides the extra control you need to match targets or fail incrementally if required. Big difference compared to using +/- intensity.
Not sure how to best implement a toggle, but it would need to be quick and easy. Perhaps tap to move to next mode from AUTO (which is Slope) and a quick double tap moves it back?
If that would be awkward how about an Option called Toggle Mode (OFF/ON). When enabled the only two modes active are AUTO and Slope. I would use this for any tough looking HIIT workouts I wasn’t sure about. If you run into trouble – TOGGLE!
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Seconded. I’ve use Erg mode for 6x8-minute sets of microbursts where you alternate between 15 seconds at 140% FTP and 15 seconds at 40% FTP with no issues. The whole workout is slightly shifted to the right, but the intended training effect is there and I’ve never had an issue. I suppose your mileage may vary based on equipment.
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