Rapid Pulse - What a load of crap and some terrible advice. The problem is almost certainly your Snap. I’ve been through 2 Snaps so have some experience with them.
The first lot of Snaps had problems with power accuracy. If you did the advanced spindown on an ios device you might have been lucky to get it calibrated correctly. Android users like me seldom got it to calibrate properly. The effect was that power accuracy was way off what my power meter was reporting e.g.
150 watts on Snap = 130 watts on power meter
180 watts on Snap = 180 watts on power meter
200 watts on Snap = 240 watts on power meter
250 watts on Snap = 350 watts on power meter
300 watts on Snap = back wheel lock up.
Now when you use ERG mode on Zwift with your power meter controlling the Snap if you have an interval at FTP or higher the trainer will first go to what it thinks is the right power. It’s then a back and forth process between the power meter and trainer as the trainer settles down to the correct power. This can take over a minute and the higher the target the longer the settling down process. For example Trainerroad that I also use adjusts the power every 10 seconds and only by 5 watts at a time. I assume Zwift is similar. So if your power target is 250 watts and the Snap makes you work at 350 watts it takes 200 seconds to settle down to the correct power.
So for ERG mode to work properly your Smart trainer needs to have reasonably accurate power even when using a power meter to control it.
I’m running a new power meter that reports power the same as the old one. ERG mode on my Cycleops Hammer works great even on intervals over FTP. Where I find it doesn’t work too well is 120% intervals of 3 minutes or more that I can’t sustain. I start tiring at 2 and half minutes and if you don’t keep on top of the power you get the dreaded spiral of death where your cadence drops and the resistance gets harder and harder. For these it’s best to do the workout in standard mode where you can keep doing the interval but let your power drop as you tire.
So anyone saying ERG mode doesn’t work should get in touch with their trainer manufacturer. Something isn’t right. In my case Wahoo said my power meter was at fault. I didn’t buy this and I knew of others with exactly the same problem. I’m now very happy with the Hammer and new model power meter (power on new power meter same as old power meter).