I think the biggest downside of Apple Weatherkit is the pricing tiers.
The prices are fine, but when you're right at the tipping point between two levels (the most glaring is of course 2->5, 5->10, 10->20 million), you can't directly explain that jump. With a product that suddenly has more requests than you have planned for, you suddenly don't get any more weather data. And then to have to worry about changing the level at all is unfortunate.
Either there should be a Pay2Go model or a level for every million. I would personally prefer the former, so that you don't have to worry as much.