What are the potential problems and dangers with a ventfree fireplace?

Ventless gas burners produce humidity, moisture is a by product of burning. In cold weather the inside of windows will steam up. Most all burners now have safety shutoffs if the flame goes out, so gas is almost never an issue. Many people claim carbon monoxide, but it's not really an issue. 10 times more people get poisoned by vented burners than unvented. The only real problem can be lack of oxygen in the room. They should not be installed in small confined areas.