The tools mechanical and civil engineers use are predictable. You're confusing the things these engineers design, which have tolerances and things like that, with the tools themselves.

If an engineer built an internal combustion engine that misfired 60% of the time, it simply wouldn't work.

If an engineer measured things with a ruler that only measured correctly 40% of the time, that would be the apt analogy.

The tool isn't what makes engineering a practice, it's the rigor and the ability to measure and then use the measurements to predict outcomes to make things useful.

Can you predict the outcome from an LLM with an "engineered" prompt?

No, and you aren't qualified to even comment on it since your only claim to fame is a fucking web app