I disagree. That may be true in Haskell because it's static and the compiler finds a lot of problems.
This isn't true for all functional languages though. I REALLY want a debugger in Clojure code and it's pretty functional*.
Static typing can do a lot of good, but the stricter semantics of Haskell also goes a long way into writing code with fewer bugs.
It's because with Haskell you find a ton of errors at compile time and Clojure finds most issues at runtime.
However, there is one thing you want a debugger for in Haskell: checking when thunks have been forced.
Also, the type system can catch much more issues than "normal" compiled languages, not to mention the safety of immutable values. But there still numerous classes of issues which will not be caught by the type system. And of course, you have exceptions. Even if your code catches them, figuring out what triggered the exception can be tricky.