On many teams I have been on/worked with, a constant source of complexity is the result of poor database design, i.e. not having tables in at least 3rd normal form.
DB design is an objective activity compared most other aspects of building a web app, yet seems widely misunderstood.
Why? ironically it's BECAUSE it's so well-defined by mathematics. The language of it is impenetrable: “Every non-trivial functional dependency either begins with a superkey or ends with a prime attribute”