Well there are several issues. The fact that they do not have any connection to the flue means that lots of extra dilution air can leak in which weakens draft and cools the flue gasses. Both of these issues lead to excess creosote buildup. The fact that the flue is also usually oversized is another factor in creosote buildup. So now you have a firebox and smoke chamber with allot of creosote in it and you get a spark that lights it off. Now you have no way to shut the air down and you have allot of fuel to supply the fire. So in the event of a fire it is a big one there have been quite a few where that creosote melted and ran down and out of the firebox in a black gooey flaming mess.So what in particular makes them unsafe? I saw in the other thread that they were no longer allowed after the 70's, mine was made/installed in 86. It worked well and drafted great. I agree that it did have to be pulled out to clean the chimney. I see all the time that they are in safe, just wondering why.
Then you also have the issue of co. If the draft stalls which can happen easily with the dilution air and oversized flue ect with no connection co can easily leak out into the house