Admittedly it's been almost a year since I tried it, but:

* If you have more than 65,535 words in a document, words after that limit will be SILENTLY IGNORED (not indexed). This was 1,000 words when I tried it, which was very limiting

* Documents where your query word happens sooner are ranked above documents where your query words happen later (in the document string). You can't turn that off

* You can't get the "match" information (i.e. the position where your query matched in the document) unless you retrieve that attribute (i.e. tell the database to send you the entire document content)

* The API uses HTTP verbs wrong, e.g. POST is used to "Add or replace documents" and PUT is used to update documents (it's exactly backwards from the intended use, per RFC 7231)