I've been wearing long sleeves for a while now (I live in Sweden, it's been minus-temperatures for a while), and I think it's working quite well. The average amount of reported minutes in the sun has gone down quite quickly last month, but that can be explained by the fact that the amount of sun hours has gone down drastically as well. Especially during the hours I'm usually outside (to and from work).
Either way, I exclusively wear long sleeves at the moment. And I haven't noticed any inaccuracies. Haven't really spend too much time trying to work out if it's accurate though.
My guess is that it uses GPS information combined with the weather information to get an estimate. I haven't really done any measurements to check accuracy. But I have exclusively been wearing long sleeves for at least two months now and so far it seems OK.