They have their uses, but are limited.
• Unlike a pancake rig, the weight of your piece will pull the gun out from your body printing badly even under a very loose shirt. If you bend over to pick something up, it looks blatantly like a gun back there.
• The handle of your gun will catch on chairs and stuff when you sit down and will get pulled out or partially out, which can be embarrassing to say the least or aggravating if you scruff up your gun.
They look cool in movies, but everything looks cool in movies.
Pancake rigs or IWB rigs are a better idea for concealed carry.