My experience of it was that if you are genuinely knowledgeable and approaching being an expert in your field, it is an enjoyable experience, and just like having a (longer than usual, but not uncomfortably long) conversation with someone about a topic that you both share a deep interest in. There is nothing to worry about, as you can have a nice discussion about an interesting topic, and share interesting ideas etc.
I'm not sure if it's a lower bar for grad as such - I think it's got fewer "formal" requirements, and far more informal requirements. The most common way to "fail" is to simply not submit the thesis.
My impression was that someone failing their viva was seen as a serious failure on the part of their supervisor, who should not have let them submit without being more prepared.
That matches my limited experience with UK, although I've seen north american ones without a foregone conclusion - seems really dependent on institution.
> The most common way to "fail" is to simply not submit the thesis.
I think that's true everywhere, lot's of people just don't complete. I've heard somewhere that the "ABD" (all but dissertation) rate in US is nearly 50% at 10 years, if you go across all disciplines.
At least that's the theory - I don't know if in practice it holds up; most of the grad students and later I knew from that system came from oxbridge which has a number of quirks.