A major challenge with search in 2020 is that it's adversarial. Any open source search engine that gets popular is going to be analyzed by black hat SEO people

A major challenge with search in 2020 is that it's adversarial. Any open source search engine that gets popular is going to be analyzed by black hat SEO people and explicitly targeted by spam networks. Competently indexing and searching content is really only a small part of the problem now, with the adversarial "red queen's race" against black hat SEO and spam being the more significant issue.