If done correctly, this is not such a bad idea. The key is to avoid getting the manifold any hotter than it needs to be. I've watched the youtube videos and they are ridiculous. I just used a plumbers small propane torch and evenly heated up the whole intake manifold before igniting the crap that was inside. It took a minute or two for it to catch fire. I pointed the four intake tubes up like chimneys and let the crankcase vent tube port at the bottom supply the fire with air (240D manifold). It burned on its own for about ten minutes. There was no need to apply constant heat from the torch (which would have eventually warmed it). After the fire went out, I lightly tapped around the entire intake with the torch sparker and dumped out a load of ashes and embers. This was AFTER I had it cleaned at an engine shop (which got most of the crap off the outside so I could handle it without gloves). After I burned the garbage out i just dumped in some marvel mystery oil and scrubbed every surface I could reach with a wire brush on a bore cleaning rod. After that it looked almost like new and will remain like that because I made an new EGR gasket and forgot to cut the hole in it ... damn.

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED