Having watched both complete beginners and competent programmers try to learn Haskell my observation is that hard part initially for the competent programmer is not building the necessary mental model, but tearing down their existing mental model. People with no mental model going in tend to just look at Haskell, go "I guess that's the way things work", and get on with it. Their real problems first arrive when they then go on to try to learn for example Java which has an entirely different model.