The coach asked for very short-term cadence changes (30 seconds and 1 minute). When you lower your cadence, your trainer will need a bit of time to adjust to the resistance to the new power level. Even worse (in my experience) is if you then want to significantly increase the cadence: you have to “overpower” the trainer, which costs significant effort in my experience. You will have to overshoot your power targets significantly to change cadences that quickly.
It is much easier to shift. Yes, you will have to pay attention to keep your cadence steady and not let it drift back. But that’s just a skill you can work on.
Personally, I only use erg mode for endurance workouts. Sweet spot and up, I prefer resistance mode.