[_0w8t]

Surely a do-block is just sugar for the functional code. But that code can be used to model all imperative effects. As such the code inevitably models all troubles the imperative code can cause.

If one looks at the desugared version one can see where the trouble comes. Functional code using monadic style depends on the state of the monad interpreter that can be arbitrary complex and spread over many closures with many interdependencies. It can be rather hard to uncover what exactly is going on, precisely in the same way as with imperative code it models.