Here are some problems I've run into:
- Most people find functional programming to be difficult to understand. This means it will probably be harder for you to write functional code, and it will almost certainly be harder for someone else to pick it up.
- Functional programming languages are usually slower than a language like c would be. This is becoming less of an issue over time (because computers are getting faster, and compilers are getting smarter)
- Not being as wide spread as their imperative counterparts, it can be difficult to find libraries and examples for common programming problems. (For example its almost always easier to find something for Python, then it is for Haskell)
- There's a lack of tools, particularly for debugging. Its definitely not as easy as opening up Visual Studio for C#, or eclipse for Java.
