Ubisoft + looks like it be available on Google Stadia also in Europe, after being launched in beta on the cloud gaming service in America. Consequently, the specific subscription for Ubisoft titles should be accessible and usable by everyone.
It must be emphasized, however, as currently the official Google Stadia website still reports the declaration that the service is available in beta only and only in the USA, yet, according to various reports, the service is also active in Europe, but you need to login through the Ubisoft site and not directly from the dedicated page on Stadia.
This way you can play games like Immortals Fenyx Rising, Assassin's Creed: Valhalla and many more at no additional cost.
Once you have logged in to the Plus service account through the official Ubisoft website, you should be able to connect by going to the page dedicated to the Ubisoft subscription on Google Stadia. In this way, free access to the titles of the French publisher within the Google Stadia catalog will be unlocked, based on the provisions of the subscription.