In my ongoing quest to investigate the ins and outs of the weird new Ubisoft+/Luna/Stadia hook-up, I've come to a conclusion regarding the question of whether Ubisoft+ games can be shared via the Stadia family sharing plan. In short - no, they disappointingly cannot. Though players can share both purchased and Stadia Pro games with family members, each player must purchase their own $14.99 Ubisoft+ subscription to play Ubisoft+ games.
Allow me to show you the depths of my investigation. First, I checked out a non-Ubisoft+ game:
Very nice. There is clearly a switch there that will allow me to share Sekiro with my family members.
Then I took a look at an Ubisoft+ game: