When making hugo themes, it’s probably better defaulting to absURL and relURL over baseURL for links. It avoids most breakage on links that depend on a base and more easily allows multilingual support with absLangURL.
However realistically (for anything), interpretations of file, directory/, absoluteness, and relativity are varied. Testing artifacts for ≈ 100% internal/self linking consistency, under different conditions, and after every code change is better time spent debugging. Sub directories are exceptional cases.. even now some of my links are broken except that my web server automatically fixes common mistakes (: