Anthropic found embeddings + BM25 (keyword search) gave the best results. (Well, after contextual summarization, and fusion, and reranking, and shoving the whole thing into an LLM...)

But sadly they didn't say how BM25 did on its own, which is the really interesting part to me.

In my own (small scale) tests with embeddings, I found that I'd be looking right at the page that contained the literal words in my query and embeddings would fail to find it... Ctrl+F wins again!