When you click on a forecast you can choose the model or forecast provider, and you can set your default with an account. Windy.com just organizes the data, especially grouped by outside activity such as surfing, sailing, parasailing, but uses other data sources and doesn’t create its own forecasts.

For my microclimate, I’ve found that Meteoblue is the only forecast that doesn’t forecast the high temperature 10 degrees too low when there is a heat wave.

I've been using the Android app (with a subscription) all summer for sailing. It's good enough to help me decide 72 hours in advance which day to go out and on what body of water. Also helps determining if I'll need to have another meat bag onboard (beside my own) to keep the mast pointing up during gusts, so I have a time to make phone calls.

Windy visualizations raw or direct outputs from the weather forecast models run by agencies in the USA and Europe. These models are the "bread and butter" of operational weather forecasting, and are the baseline in accuracy for all weather forecasts generally.

That being said, you shouldn't treat these outputs as gospel. All these models will get the big picture correct, but they have limitations with respect to spatial resolution. So if you click on the Windy map and look at your local forecast, take it with a grain of salt - the model doesn't resolve down to the scale of your backyard. The status quo is for companies or agencies to use statistics or ML to "correct" the forecast against local observations, which dramatically improves the accuracy of forecasts like, "What will tomorrow's max temperature be" or, "what is the probability it will rain this evening?"

It’s as accurate as the underlying models, from which you can select the one you’d like to see visualized.

https://www.windy.com/articles/what-is-forecast-model-3397

I need to combine it with some other site like weather underground because it seems to me that windy uses past model outputs for current conditions, instead of actual observations. So it’s as good as anything for the near future, but not great for the present.

Well, it is showing Hurricane Hillary, which is on its way to Los Angeles: https://www.windy.com/?19.808,-114.170,5

It's one of the most trusted apps by sailors, so I'd say it's fairly accurate compared to most alternatives.