It's very reliable. According to the description, it "predicts what the weather will be". No ifs, ands, or buts. This suggests perfect accuracy.
That said, it's not very precise. (Precision is not the same as accuracy.)
The description isn't clear whether it shows a single effect that predicts the weather over 24 hours, or if it changes in appearance over the course of the round (6 seconds) depending on the weather. Either way, if it shows you that you have a cloudy night, that doesn't mean it will be cloudy for the entire night, or at any particular moment. So if you're sneaking into a castle, be prepared for the moon to shine through an inconvenient gap in the clouds.
Similarly, it says it shows the weather "at your location". This is equally imprecise - it could be anything from the exact point you're standing at the time to the entire region you happen to be in. In the first case, unless you snuck into the castle to do your weather prediction for sneaking into the castle, be prepared for the weather to be a little bit different. In the second case, you only have a very vague idea. The city where I live often predicts rain, but I don't often see any - it mostly rains down the south end.