I think the point was (and i'm certain my experience is) that with Apple Weather you just don't get the notifications at all (or rarely do) so it's very hard to get a feeling as to how accurate they are.
I think -- and I might be wrong, since this is from over a decade ago -- that when I first used Dark Sky, I ended up disabling notifications because it would constantly warn me of precipitation, but then when I checked the graph there was none because the model had since updated, and I wound up turning them off. So notification thresholds are probably something hard to get right, and what is appropriate for one geographic area might not be optimal for another.