I have not yet encountered a single problem (including being on the receiving end of a secondary security screening due to it) with at least 500 I have taken with an electric toothbrush in my hand luggage.

The only problem I have had was when I had it in my checked baggage - where a certain airline in Australia with the name starting with Q and ending with S crushed my suitcase and managed to crush the toothbrush case as well, breaking the electric toothbrush inside it by bending the bit that goes into the toothbrush head. I was quite impressed that they could inflict such a level of damage, annoyed as I was.