I've always heard adhesive tape can't be used as insulator, but I don't get why : I measured with a tester the resistance of a thin film of adhesive tape, and his resistance it's above 2 Megaohm.
Isn't 2 Megaohm enough to insulate even a large voltage such as 220 Volt? (-> 1 mA as current)
It depends on what you are insulating. If you are insulating a semiconductor from a heatsink, you need the proper material to both insulate AND conduct heat to the heatsink.
For "ordinary wires", insulating tape of any kind will work, but you need to be concerned primarily with fire. If there is a short circuit, will the tape burn and cause a larger fire?
Will the tape dry out and crumble away in a year? How will the tape perform in warm, cold, dry or humid weather?
Sure, wrapping a few layers of plain clear tape over a wire will work, but it won't last long and it may be a fire hazard. Use the right stuff!