I live in the UK and we usually have light rain so the weather cover is mostly brilliant. Unfortunately I recently found out that whenever there's heavier rain, the fabric part of the weather cover gets wet and water slowly leaks through to the canopy underneath so not great. Hopefully nothing will get mouldy while the wagon is not in use.
Also it wouldn't be a bad thing to add some air circulation sections in the cover as the children gets really hot under it.