I worked at a fireplace store in 90’s and we sold a bunch of the ventless fireplaces because, well, we thought they were fantastic. They heat. They heat well. But…people started to get sick and we bought every one we could back from the customers. It says it burns clean. And in a laboratory, that is probably true enough but we don’t live in labs. We live in homes with dust, carpet fibers, cloth and paper fibers, pet hair, our hair, dead skin cells, etc. all floating around in the atmosphere. A heat source draws air toward it at the floor level, and it rises as it is warmed then flows away through the house. All of those contaminants flow with the air, some being combusted. They, my friend, do NOT burn clean, and the people in the home wind up breathing in this combustion by-product particulate matter. Most do not see the soot created from using a ventless until they move a picture frame on the wall. It is astonishing how fast it builds up. Also there is the problem of water. A ventless unit burning 38,000 btu of nat gas will produce about a gallon of water vapor as a product of combustion a whole lot faster than you would think . I apologize for not remembering exactly but I wanna say 1 gallon of water per hour at 40,000 but. If Someone out there can correct/check that please? So… after awhile you will also have molds flowing through your combustion chamber, burning with the dust and messing up your health. We honestly thought we were helping people in the beginning with so much cost effective heat, but after seeing with my own eyes I would only put one in an enemies home. The unit in the pic does looks like an old ventless and it had a fiberglass back adhered to the firebrick that would have glowed quite prettily where the flame impinged. Knock it out and look for a slim direct vent unit with a sealed combustion chamber. Or go electric with flame image. TLDR: ventless bad. Soot. Mold. Knock that out and install vented/direct vent unit.

The number one complaint I see on this sub about ventless gas units is the massive amount of moisture it puts in the air. That said, my in-laws have a ventless gas stove in their rather large family room and everyone loves it. It's great to stand in front of and warm your rear after you just came in from the cold. They don't run it for long periods of time so between that and the large size of the room, the extra moisture has no apparent effect.

Edit: In summary, I'd say it's great for point of use (warming your buns, power out for a day) but I wouldn't use one for primary heat.