It's not a reason to not take on such a project and learn something. But it is a good reason to approach the subject with some humility. There are posts here every few months/weeks of someone boasting that they are running circles around Lucene in some way. BTW. Elasticsearch uses Lucene. Lucene is where all the cool stuff it does is implemented.

Implementing your own search is indeed a bit of a rite of passage. Usually, if you go look at such implementations, you'll find they implemented 1% of the features, cut lots of corners and then came up with some benchmark that proves they are faster for some toy dataset. WAND would be a good example of something most of these things don't do.

Doug is of course a search relevance expert who has published several books on the subject. So, this is not some naive person implementing BM25 but just somebody building tools they need to do bigger things. Sometimes Elasticseach/Lucene are just overkill and it is worth having your own implementation.

You can find my own vibe coded version here: https://github.com/jillesvangurp/querylight. Nice embeddable search engine for kotlin multiplatform (works in kotlin-js, android, ios, wasm, and of course jvm). I use it in some browser based apps.

If I need a proper search engine, I use Elasticsearch or Opensearch.