Every viva is “unique, not unknown” – always different, but following patterns from regulations, expectations and even traditions within departments or universities.
We can also say with confidence that a viva could be “unanticipated, not unmanageable” in how it occurs. A viva could deviate from expectations in a way that no-one could expect from the outset: a question could be unpredicted, a comment could seem random, a line of discussion could even be uncomfortable.
All of which would be unanticipated – but not unmanageable. Given the time a candidate would spend working on their PhD, investing in their development and getting ready, the viva could be surprising, more than the expected challenge, but still within the capabilities of the candidate.
Unique, not unknown. Unanticipated, not unmanageable.
Which is the short way of saying that you can have reasonable expectations, and rise to the challenge of anything you can’t foresee.
Even shorter: you can do it.