You _can_ write Haskell code without understanding what a monad is, but composing and creating these things is going to be a little painful without that understanding.

Additionally, it seems to be a harder concept to grasp than e.g. functors or monoids.

I think this can be partly attributed to many Haskell programmers first being introduced to monads that are less than ideal for understanding the concept.

Shameful plug, I've written some thoughts on this here: https://frogulis.net/writing/async-monad