Mirrors are a complete waste of fucking time in a First Person game. It requires every player action be animated in third person, as well as first. Every. Single. Action. Reloads for each weapon, leaning, jumping, crouching, walking, running, crouch-walking, picking objects up, throwing objects, taking damage, idle anims, aiming anims, talking anims...just no.
Edit: the reason Deus Ex feasibly could have mirrors is because the game is reskin-heavy. JC and Paul Denton are the same character model, for instance. Since Paul needs third person anims, JC may as well have them too. Yet plenty unique player anims needed to be made solely to accommodate mirrors regardless, such as leaning, and it's still incomplete. Waste of time that should have been spent on animating first person stuff, such as holding medkits, fire extinguishers and such. They're higher priority. And they will be addressed in the next version of true incline named GMDX.