Rare
Failure at the viva is rare. There are many, many reasons why this is the case. The three that come first to my mind are: you did the research, wrote the thesis and you know how to answer questions about your work and field.
Still, the brain looks at outliers and thinks, “Why not me?”
Remember: the viva is pressured, important, but also just the latest occasion you’ve had to talk about your research and field.