I have now used carrot for a few days, and I love the Apple Watch app! It has a great way of representing data that fits the watch so well. Unfortunately though I find the actual weather data to be inaccurate. Today Carrot has been showing me -4°C while my window thermometer and all other weather apps have been showing an accurate -8°C. Humidity is also slightly off.
I don't know where DarkSky gets their data, but it can't be from the official government operated weather station where everyone else seems to get it.
I switched to WU data in the app to see if that would help, and the temperature was accurate, but wind was off by ~2000%. Real wind was 1 m/s. Carrot now displayed 21 m/s and awarded me an achievement for high wind strength. The WU app will show this erroneous wind speed value about half the time so I'm guessing it's a personal weather station that is reporting bad data.
I love Carrot on my Apple Watch but it's unusable with inaccurate data, unfortunately

I'm guessing the chances of adding other data sources aren't huge?
Edit: I also think there's a bug. When I activated WU in the iPhone app, I also got the new data on my watch although I'm not an ultra premium member (just regular premium)