This, absolutely this. I looked over a friend's PhD program because the results were unstable. I knew nothing about the domain which was a large disadvantage,

This, absolutely this. I looked over a friend's PhD program because the results were unstable. I knew nothing about the domain which was a large disadvantage, but on the code front it was a monolith following a vague data pipeline approach. Unfortunately components wouldn't run separately and there were only a single end to end tests taking hours to run. Had each section had its own tests, diagnosing which algorithm(s) were malfunctioning would have been easier. We never did.