I am, personally, prone to the odd metaphor. “Rainbow shitting unicorns” is one I use with depressing regularity, usually in reference to a quest to find technologies or skills that patently do not exist. A unicorn is a mythical creature. A rainbow shitting variety doubly so.
Unfortunately, it seems that Unicorns now has two meanings in the tech world to refer to things that, whilst rare, do exist: billion dollar valued tech startups, and technologists (otherwise referred to as Rockstars).
The thing is that a Unicorn is something that definitely doesn’t exist. It’s mythical. Not merely very, very rare. Want a metaphor for something very, very rare? Try a Shakespeare’s Signature (nice alliteration too), or a Gutenberg Bible, or a steak in a French Bistro. Or maybe one of these other very, very rare things.
Unicorns. Not real. Not in existence.
And if you really have to go for a mythical creature in your crap metaphor, please go for something a little more aspirational than a unicorn. A wyvern Or a griffin, perhaps. Because quite frankly a unicorn is little more than a neurotic pony with an ice cream cone plonked on its forehead.
mmitII 
Hilarious Matt!