I find that using the phone app seems to update power to the display more quickly than my Garmin 530. For example, if I stop pedalling it takes about 8 seconds for the power and cadence to reflect as much on the Garmin whereas the phone is much quicker.
I think this maybe causes my trainer to find it more difficult to control the power. This activity, for example, during the second on interval my power was jumping between 295 and 310 though I perceived no difference: Xert - Activity | Supra Threshold @110%
The last interval it didn’t happen.