I would think that Arc is not yet stable enough to use for large projects. I think that the missing module system is the major concern. I am not so concerned about unstructured data because it can be mitigated with access methods and because if you are properly creating your DSL it should be pretty obvious what is going on. Also, how big of a project would you have to have before you needed enough Arc to make it confusing? The language spec is < 5,000 lines of code.