Does the data in Apple weather coincide with the data in the Dark Sky app for you? The API hasn't changed nor has its data pool and processing of that data. Should note that the only Dark Sky data being used is the 60 minute precipitation forecast, everything else is the weather channel.
Dark Sky is just a system that uses more granular data to predict the weather more precisely on a per user basis. A rain forecast for your city could take an hour+ to reach you plus a lot can change from the time you listen to the weather and the predicted time. While its needs to be managed its a system that runs itself with little outside intervention which is why 5-10 employees could run the company for the last 10 years.
Its weather data is obtained from national weather services. It takes the radar data and cleans it up, removing things like flocks of birds and such from the radar data. Now with better knowledge of storm data like heading, speed and behavior combined with your precise location via your smartphones location services it can better predict when you will have rain. Finally what sets them apart was their initiative to take the radar data and make it into an easily readable graph/chart that is always displayed. This isn't anything you couldn't do yourself with a bit of education on the subject, it would just be a massive pita keep the data updated.
It was likely less expensive for Apple to buy Dark Sky then to supply it to all their users. Every update via their API would cost the developer using it .0001 USD. Now with widgets and watches this data is being refreshed A LOT.
Point is, for it to be less accurate now than it was before would mean its data isn't being handled somewhere and you are seeing a different areas forecast (guessing?). I've checked a couple spots in the UK (vague I know) and it was the same rain forecast data in the iOS native weather app as it was in the Dark Sky app and other apps on iOS and Android that use the Darksky API. While I can't really know the true weather conditions in those locations they all seem to to be using the same precip data as always.
EDIT : Darksky.net still has there weather data on their website to cross reference as well.
