I have two in two houses. I use them for emergency or temporary heat. They work well in that setting. Be aware that the vent CO2 (NOT carbon monoxide) and water vapor into the room so you don't want to use it as a primary heating source. They also shouldn't be used in bedrooms. I haven't had any issues from soot - that is the byproduct of incomplete combustion which they are designed not to do.

I put one in a house for sporadic use and then rented out the house to hillbillies who ran it all the time, which created moisture and mold problems. It is amazing how much water is produced burning natural gas or propane.

Thank you @sktn77a and @Seabornman for the first-hand and useful information! I really appreciate it :-)

You couldn't give me a ventless fireplace. We have a second house with one. They deplete the oxygen in the house and when operation they emit lots of water (probably not an issue in AZ) carbon dioxide and other small amounts of combustion by products. while I don’t normally suffer allergies there are three things that cause a slight asthmatic like symptoms in me: dogs, mold and ventless fireplaces. These units all have oxygen sensors which is a primitive electomechanical device which is supposed to shut down the heater when the O2 level drops to low. When not operating properly they can like any combustion device emit carbon monoxide. When we rented the property we had to disconnect it beacauss local codes prohibit these in rental properties. I believe most building codes near me prohibit new installs in any property .