Consider yourself lucky then, or perhaps Wahoo are better at doing this than the rest. I know that the Kickr is one of the better units out there (although not sure about the Snap).
I also know that most smart trainers simply estimate the power based on the resistance applied and the RPMs of the roller. I reiterate, this is just an estimate, and there are too many variables in the equation for it to have any real accuracy (tyre choice, tyre pressure, how tight the wheel is pressed against the roller…)
In my case, my trainer reports about 50w extra when I’m doing 100w, and about 25w lower when I’m pushing 250w. Fine if you just want to pootle around Zwift, but not good if you want to do structured interval workouts.
Yes, of course you can use the trainer without Erg mode, but that defeats the purpose of a smart trainer in the first place. If I wanted to do that I’d dig my Kurt Kinetic out of the basement (solid unit that it is)