I have recently done a 4dp test on Wahoo/SYSTM, to compare my results:
Peak power 970 (Xert) / 927 (Systm)
TP 269 (Xert) / 249 (Systm)
… so close but at the moment Xert is estimating higher. Part of this I think is that I use Systm on my turbo (Saris H3), whereas Xert has data also from my bike (Stages l/h crank) which seems to estimate a slightly higher power, possibly on account of loses in the drivetrain between the crank and the rear axle. I am also not the best at running regular calibrations…