unvented gas heaters, natural or LP, are over 99% effective when operating at best.
Downsides, the danger of not operating correctly and producing carbon monoxide so make sure you have a working CO detector/fire alarm. Other things to be aware of, unvented gas heaters add a lot of moisture to a space. If the space is well sealed, moisture build up may actually become a problem. If the area isn't insulated and sealed well, or if the space is larger than the optimum for the size of the heater, the moisture can actually be a bonus since winter indoors tend to be dry.
Last numbers I saw for southeast US, electric cost was below gas, natural or LP, and below oil. I know my LP has gotten high enough so that when I replace my system I went with a heat pump with electric strips rather than a heat pump with LP backup as I was originally planning. I am considering placing a small, probably only 10000 BTU or may 20000, unvented wall mounted heater in the largest open space. It would allow quick heat rise if I needed it with the heatpump/electric strips unit. It would also give me emergency heat enough to keep the house lines from freezing during a winter power failure episode.
My decision if between IR gas units and blue flame units. An IR unit is design to radiate infrared heat and heat objects/people in front of it. A blue flame unit is design to heat air and distribuite it, either through natural convection or an auxilary fan. Some units use a fan normally but will function without electricity if needed to do so.
Bad winter storms aren't that common around here, but a storm in a rural area involving ice or wet snow can cause power outages. If you're near the end of a line, it can easily be a week or two before power can be restored to you.
Couple having emergency heat available if needed coupled with the use as an auxillary source for large quick heat rises if needed and I need to install a unit. I still have the tank from the old system with over 200 gallons of LP paid for. Got some estimates and it would run me about $300 to $400 to install a 10K or 20K unvented heater. I need to replace my hall smoke detector anyway, its over 10 years old as far as I know. I'll replaceit with CO fire combo anyway. The current recommendation is one CO on each floor, but I plan as I replace smoke detectors in the future to replace them with CO/smoke detectors. Cost is an issue, and the recommended replacement for CO devices is only 5 years vs. 10 for smoke detectors. However, I saw a CO detector ad recently guareenteeing a 10 year life. I need to check to see if the newer models are considered good for 10 years.