They aren't.
They use non-fossil fuels and in the right environment, a well maintained log burner is the best option.
I live rurally, there is no gas in our village and I have a large garden. I prune and stack my own wood all year in a wood shed so come November, it is dry and good to burn.
It involves no fuel miles, no carbon footprint at all. And it's free. It I didn't stack it, I'd have to drive it to the tip using diesel to get there, or I'd have a bonfire producing the same amount of smoke but benefitting no-one and meaning I still need another fuel for winter.
My nearest neighbour is 40 yards away. I can't smell their log burner and they can't smell mine.
Environmentally they are as sound as it is possible to be. Your views may be related to the particles of smoke perhaps in a town or city, particles that can be harmful to human health not the environment. But out here in a low populated (and very exposed) area, they do no harm, and are returned to the soil with the next fall of rain.