"Bed of roses" seems like a weird idiom because roses have thorns.
Feel free to read it as "Haskell isn't all sunshine and rainbows."
To be clear (before you say that "sunshine and rainbows" seems like a weird idiom because sunshine causes cancer), I meant that I think Haskell also has disadvantages.
https://en.wiktionary.org/wiki/bed#Etymology
from Old English bedd (“bed, couch, resting-place; garden-bed, plot”)