Expressiveness isn't the problem. No doubt it can be less efficient and re-usable but constructed languages are mostly syntactical interfaces to subsets of the semantics we express in natural languages all the time. By using them we can precisely express combinations that can be difficult to convey in natural languages without losing time to perspecctive and interpretation but that can only a problem if you're in a hurry. In most cases, it'd be far more limiting to have to express natural semantics in a constructed language than the other way around. What's more is that all the orimitives of these languages are developed using the same strategies we use in natural languages. It's just the morphs and structures that differ.