Stakes and challenges
You definitley are well aware of Elastic Search outstanding performances when it comes to storage, indexing, filtering, and data queries. Now all the operations you were performing on a daily basis are doable within Fast2, as well as storing/updating content and metadata or retrieving them from ES data centers.
On the techier side...
Fast2 decided to stick to the Java RESTful search engine to deal with nodes or data stored on disk. Being provided directly from ES, this engine leaves no room for compromise, performance-wise.