Anecdotally, if you have time - I find if you wear glasses of quite a strong prescription, the e-gate card is a preferable option to the facial/eye scanner, as when you are in the smart gate you only need to present your two thumbprints to get through instead of staring at the black box for an interminable amount of time. Similar story for leaving DXB, but then my last trips have been within the window that DNRD computers updated so I had to see an officer anyway!