Assuming you’re on the road they should be reasonably accurate , well within the error above.
This is from the last 90 min road ride with a power meter
………………………..avg power…..Work Done……Kcal
Garmin…………….214…………….1210kj………….1277
TrainingPeaks…214…………….1207kj……………1207
Strava………………215…………….1212kj…………..1352
They all pretty much agree on avg power and work done (which you’d expect)
There’s a bit more variability on calories but all roughly in the expected ball park.
Going back to the OP 1500cals over 100minutes would need an avg power of ~ 258 Watts, which is ~ 22mph on the hoods on a flat road. So if the OP did 35 miles on flattish roads, then the figure would be about right.