This isn't my experience at all. Imperative code with side effects is harrowing to debug because it may do different things depending on what execution path got you there.
Pure functions are easy to debug. Run it, check the result against expectations. Step in if necessary.
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).
Pure functions are easy to debug. Run it, check the result against expectations. Step in if necessary.