Privacy coins may have to wait for quantum penetration testing. As is anything that is centralized or encrypted may only be as safe as the next innovation or hacker. Steem has mechanisms for privacy as well(memo system) but the nature of this blockchain relies on openness in general. Systems with multiple points of view aren't as secure as single POV systems like the directed acyclic graph... IPFS is one such system that relies on distributed hash tables, but these tables aren't updated at the same time which allows some conceivable vectors of attack though I'm not aware of any success. Blockchains make trust ubiquitous, trust is the fungible commodity we wish money was. It's value to us can not be understated. I will look further into holocoin, but my initial belief is that EOS (a multithreaded DAG) will provide the first solutions that holocoin seeks to answer which may leave it in the dust. I'm also less trusting of hardware dependent software after seeing the intel chip exploits.