Depending on the country, the PhD defense is more or less ceremonial. If you have made it to the defense, you can pretty much not fail anymore.
In my program, many of us would strategize for the 30-60 minutes of the closed door grilling. We sought to give our committee members obvious things to criticize with the PhD student having prepared arguments to defend against these criticisms. E.g., I ashamedly included quite a few spelling and grammar errors in the first few pages of the summary section of the thesis (the only part anyone would actually read) and we spent at least 15 minutes on my horrible writing ability.
In general, the main outcome of the closed door portion of the defense was requests for additional work. It was common for committee members to suggest additional things that could “improve” the thesis work. Not surprisingly, many of these suggestions involved applying a committee member's methods, even if not plausibly applicable, so that one would publish another paper citing the committee member’s work. Some students, including myself, would have a job lined up before the defense to timebox the amount of additional work that could be requested.
"I don't trust your maths" "I don't feel this analysis is right, but I can't describe in what way" "you are clearly not very knowledgeable" and many other similar things.
Asking me a question and then before I can open my mouth answering it yourself, and then insulting me for not answering it was the start of the viva defence and it set the tone for the rest.
I was also heavily criticised for not having cited a paper that came out in-between submitting my thesis and the defence, despite this being literally impossible to have done so, and, despite having already had gotten a job in that time, was given limited time to do additional experiments, write whole new chapters, new code, do new experiments, etc. Ended up adding 90 pages of material to the thesis.
In the end I had to quit the job I had just got, because It would have been impossible to not fail my PhD program and keep the job.
Afterword's, in behind closed doors discussions it was revealed that one guy had pushed for almost all of the required extra work deliberately to try to make me fail, because I had done something he could not.
I mean, personally, I don't really care about reputation of academia, but given that you described a horrible story where somebody basically tried to ruin your career, and given that you decided not to name specific people or institutions, it seems that the whole (UK) academia will have to take the reputation hit for the alleged scandal.
In the first half I was privately grilled by my committee. They wouldn't let the dissertation go to a public defense until they'd satisfied themselves that it was fine.
As far as I can tell the public defenses usually look ceremonial at GMU (at least in its engineering department), but they aren't actually ceremonial. Anyone from the public can ask questions at a public defense, so they aren't ceremonial. However, the goal of the (first) private half was to try to make sure that the defender is ready for arbitrary questions (because he understands the material). So it's unusual for the public to ask questions that the defender isn't able to answer. I got some questions I hadn't heard before in my public defense, but I was able to handle them.
You can see my public defense here: https://www.youtube.com/watch?v=QYH18NpsRu8
So that even if people don't fail their defenses often, there might be many that didn't get theirs scheduled at all? This is just speculation on my part.