>I saw a junior engineer try to add a patchwork of libraries just because he was not confident in his ability to do something correctly, leading to a lot of maintenance overhead that grew very quickly.

That's why they're called junior and they get paid less. They're meant to be supervised to some extent. If the problem is only caught long after the damage is done then the issues run deeper than the capacities of a single junior engineer. Junior is just a nice way of saying inexperienced. Experienced != good, but inexperienced almost always means some degree of bad.