Great question — answering as the UphillTech team:
It isn't the heat that's the problem. The problem is the fact that those narrow strips of aluminum "needles" can work their way into a lamp socket (or the extension socket at the end) and make part or all of the tree "live" at some voltage that depends on exactly where along the string the contact occurs. AC-powered LEDs are just as dangerous in this regard as other kinds of lights. At a minimum, ground the structure of the tree, and plug any lights into a good-quality GFCI. This should catch any single faults before they have a chance of becoming a double fault that could become a fire hazard. Better still, just stick with the color wheel.
Full answer by Dave Tweed: https://diy.stackexchange.com/a/129365 (CC BY-SA 4.0).
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