unvented gas fireplace insert

My fireplace insert is smelly and the mainly orange flame indicates that it is not very efficient. A couple of years ago I took it to a repair person on Colon just north of the caraterra which resulted in little improvement.
I am now considering ordering a new unit on Amazon.
Since gas stoves burn very efficiently I am having trouble understanding why it is difficult to get a similar efficiency in a unvented fireplace insert.
I welcome any comments.

We had a gas powered wood burning fireplace, but when that became impractical, we bought an electric "fireplace" made in China and it's been amazingly satisfying. I used some beige acrylic paint to cover some of the bright red bulbs that came with it. The best thing is that, although the fireplace never smoked, the chimney is no longer sending any smoke back into the house from its top exit. Also, it is saving the cost of wood. Just a thought.