When you say "you haven't had much problem" one can only assume you're _not actually reading the output_. In fact, like most things in modern times, one has to assume you arn't actually reading the output. You're skimming it; you're finding what makes sense and extrapolating that. This is the 70%.
The problem with non-deterministic models is that the output can't be deterministically assessed. You're harboring a delusion that you're getting real good output.
Most likely you're doing the baby extrapolation: you make it do a small, tightly scoped project and it's does 99% right. Just like a baby doubles in size in a year. Extrapolating, that baby will double again; but it doesnt.
Your human compensation limits does not extrapolate to the size and knowledge that's fed into the model and the context it extrapolates.