For a TV? I've mounted many TV in the Mattamy niches.
You have to be very careful with the size of TV and bracket that you get to fit in there.
I'd almost recommend getting one. In this room I would recommend putting the TV on the right wall.
Building one above a fireplace that wasn't structured for it in the first place may not entirely be possible. You need to create a cavity and in order to do such you need to have enough space in the bump-out on the back of the house to fit it. It may not be high enough, it may not be deep enough, it may be structured differently.
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