it really depends on what price and where you are buying it from.
If you buy herman milller chair refurb firstly there is no 12 year warranty from herman miller, even if you buy from their outlet store a refurbished chair or buy a chair from someone else. For example a Embody gaming chair is appx £1100 on their outlet store. Its not worth that price because you loose the warranty, might as well pay the extra £400 for the warranty, they usually do 15-20% off quite regularly so wait for one of those price drops and pick one up.
if however you buy something like a refurbished Aeron for £600-£700 price range then its worth the risk. Very rarely do their chairs break. I don't like mesh chair so bought a embody gaming chair, however the aeron have a much nicer recline/tilt mechanism