Is the viva unfair?
It might help if you knew what your examiners thought about your thesis before the viva.
You could benefit by knowing what questions would be asked, or how long it was going to take.
You might feel there are certain things you would want to control about the situation.
Or perhaps you really wish that during your research more stuff had happened the way it was supposed to.
Even taking all of that into account the viva is pretty fair.
You get several years to do the work and one opportunity to have a conversation with your examiners. The process isn’t hidden. There are regulations for your institution and expectations more generally: if you don’t know, you can find out. There are kinds of work that help someone get ready, and they’re not too onerous: if you don’t know, you can find out and then get ready. If you need more support for any reason then it will be there. The viva is a challenge. It could be difficult – in a way it’s supposed to be.
But it’s also fair.