It's frowned upon because the imperative subsets tend to code that's less general and harder to reason about. These critiques falls identically on Java, though nobody misses it there because nobody practices the generality and reasonability available in a very simple typed lambda calculus there.

It's remarkable how much more you have to keep alive in your mind to program in Java (or even Clojure) over Haskell. The reason things are more complex is because you're freed from having to think hard about state and execution methodology except at bottlenecks of design and performance.