The pricing is tricky (the per-GB price is cheap, but the retrieval can get horribly expensive). There's the fixed 4-hour delay for all actions (including listing stored files), which makes any interaction a pain. And there aren't really any good clients or high-level libraries that abstract away this complexity.
For a disaster recovery, I would certainly go for something simpler and easier to use. When everything is one fire, the last thing I need is dealing with a tricky API to restore the company files.