Xapian is to Cyrus what Lucene is to Elasticsearch - the search engine embedded into the larger server.

For me it would be the use of Java. I've just had too much bad luck with it. Admittedly that's not really very objective reasoning.

From memory (sorry, it was a while back) we actually started with Elasticsearch, but it was way to heavy and looked for an alternative solution. Even with a single user it was consuming way too much memory.

Sorry Alfie, your memory fails. We started with Sphinx.

We use Elasticsearch elsewhere (ELK stack), but not for mail search.