I used it in the past with the Freebase dump and was amazingly stable (also two years ago). I was using LevelDB on a pretty beefy machine. The big issue was the time for loading the data. At the time the MongoDB backend wasn't good enough. I posted my experience in a response in the thread.

I see Cayley now supports other backends beside LevelDB. "PostgreSQL and MongoDB for distributed stores" - that's good to read.

is there a description of the RDBMS low-level model? Is it something like a single s,p,o table, with indexes (s, so, spo, , po, etc)?

Exactly so, for the RDBMS model. Yeah, this has it's own issues, but it's the most direct method. We do a little extra trick with the indexing; joining on fixed hashes instead of the full value, but nothing crazy.