It is scalable enough because you need to download log(index_size) rather than full index.

The use case - any private data which you'd like to encrypt on the server while preserving ability to search. Medical records, financial data, emails are some examples.

Easier to say what this database is not for. Big data applications where you need to use a significant portion of data in the db (like 10%) in map-reduce queries is probably not what ZeroDB would be practical for.