It’s not a mess as in spaghetti code, which you will find with novice programmers. It’s a mess as in complex and disjointed codebase. Happy path works somewhat, but it crumbles if you run it long enough or encounters an edge case.

You need a very good level of insight to build a codebase to do what is supposed to do, to not do what it shouldn’t do, and to still be comprehensible. Because a software is a system and building system well is what engineering is about.