My opinion is that a very small portion of all those triplestores is really production ready. We tried some of the most popular, and only Virtuoso came half as way to be treated as a viable solution (in terms of stability and predictability of performance). But as for the performance, mostly writing, it was not really that good. Other mostly have bugs and issues which make them unusable.

Also, DB people are used to have some lower level things in their toolbox, while with these you really don't have an idea what's happening with data structures.

A fair portion of these database is not even maintained today.

Just to conclude, it's very hard to find an alternative in the triplestore world to Neo4J, Cassandra, MongoDB, Couch, <put_your_db_here>, (especially free or cheap) which developers can just get up and running easily and experiment, learn and scale latter.

Documentation and community support is another topic altogether, don't make me start on that one...