You need to look at the requirements of your TV mount.
If you're mounting into the 1.5" depth of a stud on the flat instead of the 3.5" depth of a stud on edge, your mounting bolts don't have as much wood to bite into and won't hold as much weight. If you have a big mount for a big TV, it may call for lag bolts longer than 1.5".
You can, of course, mount it with shorter bolts, it's your TV and your house, but if your bolts aren't long enough (as determined by the manufacturer) to support the weight of the mount and the TV, it may lead to a sudden, unplanned meeting of TV and floor.
When you mount the TV up high, you're going to be looking up at it. If you have enough recliner chairs for everyone likely to be watching at your big Superbowl party, that's probably not an issue, if not, somebody's going to have a sore neck from the uncomfortable angle. If you have a mount that tilts, you can, of course angle the TV down so you're mostly withing the viewing angle that the screen will look good at, but you're still looking up at it.
Additionally, if there are any lights above the TV, or even anywhere on the ceiling between the seating area and the screen, they'll be distracting at minimum and possibly causing glare/reflections on the screen.
The heat from the fireplace will not do the electronics in the TV any favors. Of course, some people seem to "upgrade" to larger TVs with more features on a somewhat regular basis, so you may be replacing the TV long before it's worn out, even with the accelerated wear caused by the additional heat. This may or may not be an issue for you.