content: The chief problem here is that when a bit of data is stored with a very small energy investment (like a single molecular bond), then that bit is vulnerable to being flipped by very low-energy events like cosmic rays.  One of the main causes of genetic mutation is cosmic rays hitting our DNA, and presumably DNA storage will be similarly vulnerable to bits being randomly flipped in the same way.  You'd need a high degree of redundancy to deal with this, which makes it somewhat less efficient than it says on paper.