I don't think Ethereum is any risk to BitShares X. I think the one-chain-to-rule-them-all model is going to unnecessarily slow down any currency or decentralized exchange DACs (or DAOs/scripts/DApps) since validators need to validate everything else happening on the blockchain. BitShares X runs optimized compiled C++ code purely for a single purpose: a decentralized bank and exchange.
Ethereum is interesting as an easy way for developers to launch new DACs that can interact with one another. And for that I am glad it is being developed. However, I still have some problems with the design, and I'm not just talking about POW. Even if Ethereum were to adopt DPOS, I don't like the non-concurrent imperative programming paradigm their design has adopted. I think it would better if it were designed around the Actor model
instead. I would like to see separate validators for each DAC on the blockchain which have nothing to do with the delegates of the master blockchain. Then each DAC validator can evolve the state of the DAC at its own pace independent of the evolution of DACs they do not communicate with. I would like to see BitShares X exchange features built-in to the master blockchain so that BitUSD can be used as the "gas" as well as the funds for the DAC reserve which gives the DAC-internal tokens value. Finally, I would like to see more flexibility in the programming languages and VM environments used to implement the state evolution of each DAC. In fact, I am wondering whether it makes sense to just use a PNaCl
sandbox to safely run any arbitrary native code that the DAC developers provide to implement the DAC business logic. I plan to write more about my thoughts on this later when I think through them more carefully. But the point is Ethereum (or more generally the concept of a meta-DAC that manages multiple inter-communicating DACs) is interesting, but I don't necessarily think Ethereum, as it is currently designed, is the way to go with the concept.
I haven't looked into Bitcoindark or Emunie yet to properly judge them. But at first glance, it doesn't appear to be anything to worry about.
There is not enough information publicly available yet about how NuBits solves the volatility issue and how it is any different than BitAssets. Until they actually have something substantial released, there is no point worrying about it.