From what I've seen, a lot of API developers find that openapi-generator flat-out breaks or produces broken code for their OpenAPI spec (varies language-to-language of course).
Beyond hitting the "it just works" mark more often, some other key differentiators:
1. much easier to configure & customize
2. auto-retry with exponential backoff (this is huge for our customers)
3. auto-pagination
4. overall ergonomics of using the SDK
5. internals that look handwritten (some people care, others don't)
6. one-click releases to github & package managers w/ semver, changelogs, etc
7. careful handling of edge cases (eg, will adding a new enum variant cause your Java client to crash?)
8. a long tail of more advanced features, like webhook signature verification, streaming, etc
Thanks for the question – hope this helps :)