If you're on trial for doing X and your jury is told by a prosecution witness "mrkramer did X" and under cross they admit that's based on computer records which are often bogus, inconsistent, total nonsense, it doesn't take the world's best defence lawyer to secure an "innocent" verdict. That's not a fun experience, but it probably won't drive you to suicide.
One of the many interlocking failures here is that the Post Office, historically a government function, was allowed to prosecute people.
Suppose I work not for the Post Office (by this point a private company which is just owned in full by the government) but for say, an Asda, next door. I'm the most senior member of staff on weekends, so I have keys, I accept deliveries, all that stuff. Asda's crap computer system says I accepted £25000 of Amazon Gift Cards which it says came on a truck from the depot on Saturday. I never saw them, I deny it, there are no Gift Cards in stock at our store.
Asda can't prosecute me. They could try to sue, but more likely they'd call the police. If the police think I stole these Amazon cards, they give the file to a Crown Prosecutor, who works for the government to prosecute criminals. They don't work for Asda and they're looking at a bunch of "tests" which decide whether it makes sense to prosecute people.
https://www.cps.gov.uk/about-cps/how-we-make-our-decisions
But because the Sub-postmasters worked under contract to the Post Office, it could and did in many cases just prosecute them, it was empowered to do that. That's an obvious mistake, in many of these cases if you show a copper, let alone a CPS lawyer your laughable "case" that although this buggy garbage is often wrong you think there's signs of theft, they'll tell you that you can't imprison people on this basis, piss off.
A worse failure is that Post Office people were allowed to lie to a court about how reliable this information was, and indeed they repeatedly lied in later cases where it's directly about the earlier lying. That's the point where it undoubtedly goes from "Why were supposedly incompetent morons given this important job?" where maybe they're morons or maybe they're liars, to "Lying to a court is wrong, send them to jail".
