Most of the concerns are about the heat but most of the heat comes out the front of the fireplace and if you install a decent sized mantle the heat rising is deflected and I actually like T.V mounted over the fireplace to keep from having to deal with 2 focal points and if the room has the space to move the sofa back as far as possible your neck does not get a crick. In a perfect world I like my fireplace in my LR and the T.V in the family room to avoid all this completely.

In addition to the heat, often fireplaces (even gas ones) will have brick or stone that can be difficult to work with when you are trying to hide the wires. Most fireplaces also don't have a great place to put your cable/dish box, blu-ray or roku or other accessories people use with their TV. Having these located significantly away from the TV means that the clicker has to be repointed at the item you are controlling instead of having a one-click solution for turning everything on and off.

If I have a 48 inch fireplace can I put a 64 inch television above it