I've found they vary quite a bit, both in the data points they provide, and the depth of data. Especially comparing free vs paid services. Some provide historical data, others want lots of extra cash for that. Some provide hourly and even minutely results (like my current favorite -- forecast.io), and some do not. And the underlying conditions they provide can vary. You might get simply 'partly_cloudy' from one, an actual cloud cover percentage from another, or even the cloud cover at different altitudes from a yet another provider. So you'd have to do a lot of normalizing between providers to come up with a common API.

So it certainly could be done. And I actually looked into this a few years ago, but it wasn't trivial enough to make it worth the time back then.