A simile is a less forceful use of figurative language than a metaphor is - it is easier to believe that a heron is like an undertaker than to believe that a heron is an undertaker - which some people take to mean less good, or less powerful. It's worth holding in mind that a poet may want to create the subtler effect that a less forceful tool achieves, to use a comparison that draws less attention to itself, or to maintain that, although these two things are alike, they are not identical. In the case of 'Machines', by Michael Donaghy, the conceit of the poem is introduced as a simile, in a way that draws attention to the act of comparing them and the cleverness that maintains the comparison of two disparate things.
