Heels or better shoes with heels are designed to have heels. That sounds like an open door but it’s crucial to the design of that sort of shoes. Slippers, sneakers, loafers and other types of shoes are designed without heels or destined to have at most a heel of an inch.
But flats…. that’s a heel without a heel, a cheap solution for a design problem. Just leaving out the most essential part of a well designed, beautiful and stylish shoe can never result in something beautiful or functional for that matter.
It just doesn’t work, it’s not acceptable, don’t make them, don’t wear them.  
 
The only exception perhaps are boots…. They can work with or without a heel… but otherwise; stay away from flats.