Automail didn't have the fine control human muscles had. This could be largely attributed the the fact that human skin has millions of nerve endings that feed information to the brain. Then the brain could transmit signals to the skeletal muscles on how they should move.
Especially for the hands. Artificial muscles and steel could not replace the level of sensitivity real muscle and skin had.
Automail is moved with a conscious effort, but one often forgets to be careful with metal limbs. The best automail limbs might give the full range of motion a real arm or leg would, but without the necessary input and warning signals such as pain caused by injury or heat and cold.
That's why automail could never replace the real thing.
Most especially during sex; as Roy Mustang testifies.
It is advised that Automail be removed during copulation as the amputee may injure or maim the partner. More often than not, such 'accidents' would lead to acute embarrassment for one or both parties; wherein a trip to the hospital emergency ward is needed.
Roy fervently hopes that such a trip is not needed.
"I'll get the scissors...OW!" he suggests to Ed, whose automail fingers have somehow entangled themselves in his pubic hair.
"It's your fault you bastard." Ed breathes, trying to use his free hand to open the dresser drawers in search of the elusive scissors.
"Why is it mine?" Roy winces as he feels a sharp, insistent tug in his nether regions. "You should know better."
Ed crows triumphantly, shears in hand. "You told me to take off the gloves, dammit! Tired of glove-sex, yeah right. Ask for another hand job and IT gets cut off too!"