The biggest spoiler of 'A Beast Tamed by Villainess' is the revelation of the protagonist's true identity. It completely alters the way you view the story. Also, there's a surprising turn of events in the final chapters that you wouldn't expect. However, it's best to discover these while reading the book.
The Genshin manga can potentially have spoilers. It might show events or details that haven't been encountered in the game yet. It's best to approach it with caution if you're worried about spoiling your game experience.