I think this comes down to experience. My favorite programming language is Agda and I have a lot experience writing code in Haskell and Lisp variants. I personally also find it trickier to debug in functional languages. I believe this comes down to how you debug: I am comfortable using a debugger for debugging but I simply find it easier to debug using prints (for context switching reasons) but this is not always an option in a purely functional context without any IO. This does push me out of my comfort zone sometimes, and I would say it's accurate that I debug much faster in imperative languages. Even though I write better and safer code in functional languages (and even though I like them better).