Does it scale is a very valid question for Cayley. Running sparql.uniprot.org I know virtuoso scales easily to 27 billion triples. Best benchmark I have seen for Cayley is 21 million RDF triples.
RDF and relational are similar but free form RDF is hard to put into a pure relational schema, as schema is derived from the data in RDF. There are ways to recover relational schema's from RDF but these are not yet in production state.