Private blockchains Vs Hashgraph, Ripple, BigChainDb

Most distributed databases in existence today are fault tolerant (so they keep running if a few nodes go down among many), but not Byzantine fault tolerant (BFT), i.e. they can't handle arbitrary faults. Most blockchains are BFT, so even if you don't trust some of the nodes, you can trust the overall system.

Most distributed databases allow the change or deletion of data. (There are some interesting exceptions here, but I digress.) Most blockchains do not.

Most distributed databases have a central "admin" user who can do almost anything, including change data, delete data, delete entire tables/collections, or delete the entire database. Most blockchains have no single point of control like that.

Until recently, BigchainDB wasn't BFT and the database admin (e.g. the MongoDB database admin) could cause havoc. The next version will be very different: it will be BFT, and it won't have any single point of control (i.e. no more global admin or anything similar).