I have no idea what you are talking about, but the reason they scare me is because they provide an opaque mechanism for trading that enables or allows the unmanaged exposure of major financial institutions. This means that the confidence that anyone can have in a particular counter party in a transaction is undermined - whether or not they are participating in these transactions, because we cannot know.
Most markets are regulated in such a way that unlit venues have to report executions back to the lit market. Also most alternative venues will operate through clearing houses (the same as lit public markets). The risks you mention are real but as mentioned most sophisticated markets have regulations and rules in place to mitigate them - after all a darkpool or crossing network that failed to find quality and non flakey contra parties doesnt present a compelling case for its use!
