Relatively few vivas are unchallenging, but relatively few are overwhelmingly unfair.
Relatively few vivas result in major corrections – and not too many result in no corrections either.
Relatively few candidates are unprepared; but there’s also not many candidates, relatively, who are able to respond perfectly.
(in fact, probably none can)
Relatively few examiners could match you for your knowledge of your work; are you one of the relatively few candidates who could match their examiners for general experience of academia?
When we consider all of these things together, we can see, I think, that relatively few of these are actually concerns for the viva. They don’t matter – in terms of you doing well, of you passing, and so on.
Relatively few vivas result in failure. There are many reasons why.