About 6 or7 years ago, I helped UL investigate a case on Fedder/Sears inwall electric heat/air units, after several fires were caused by them, I was involved with one home and a guard shack fire.
These units had an electric heating element right behind the out vent grill, the only protection was a automatic resetting thermo limit that would reset it's self, the housing around the heating elements was made out of a flammable ABS plastic, the heat element was dependent upon the blower motor in the unit which had a double shaft that also turned the fan blade for the condenser air, if at any time this blower was stopped I.E motor failure, fan blockage, or even if the selector switch was put partway between selections, the heating element could still operate and would heat up till the automatic hi limit cut out, at this time it would cool and turn the heating element back on, each time this went through a heating cycle the plastic housing would soften and droop a little closer to the elements, and after enough cycles the plastic housing would come into contact with the heating element and combusted in to a fire.
The problem with identifying the cause was because this burning plastic would drip down to what ever was combustible below the unit causing the fire to appear as it started below these units, and burned up to the unit, after a demonstration of a new unit by putting the selector in the middle and letting the unit catch fire at a local fire house safe burn room, they finely put two and two together, CPSC required a recall of these units and a few other brands that used similar automatic resetting high limits.
So should we always trust how safe things are just because it has a UL label?
I don't, but at the same time I can't afford to check out everything, but if it looks unsafe I look a little deeper.