Dark Sky was eerily accurate, sometimes to the minute. That was great because my dog hates rain. I've often dropped what I was doing immediately and get out earlier because of a Dark Sky notification. If it told me I had 25 minutes before rain started, I really had 25 minutes.
I've noticed that it got progressively less accurate the closer it got of the shutdown by Apple.
The iOS weather app is pretty inaccurate and borderline useless in comparison. It will tell me it's raining when it's not and vice/versa. Time predictions are so out of whack that they might as well not be there; they also get revised all the time.
I understand that weather can be finnicky and predictions are just that. But Dark Sky did it correctly and Apple does not. I wonder what was the point of acquiring them, if their UI is inferior (although pretty!) and so are the predictions.