It's most accurate on loops, circuit rides, which negate differences due to wind and elevation.
Same as the various online calculators for power/etc. These work best when wind is factored into one-direction segments and routes. Lends a dose of reality to my handful of top tens on Strava. Factor in the tailwind and my 300+ watts over 15-20 minutes turns out to be closer to 100 watts.
I've compared my Strava and online calculator power guesstimates against other folks my size who do use power meters on the same routes in the same or similar conditions. Close enough for my purposes. If it's off by 5-20 watts, no big deal. Until I can generate 200 watts over a favorite 5 mile training circuit, I don't need a power meter to remind me how wimpy I am.