It caused a heavy buzz on various amplifiers even when used correctly, so it's either poor quality, bad earth, or internal misconnection of some sort.
It's potentially damaging, but at least it shows up on sound equipment so you can stop using it - it may not show on other types of gadget.
The whole figuring out the negative polarity thing wouldn't be straight forward for everyone either, and the adaptors don't fit well or even very firmly, so it sounds like a neat idea, but it doesn't work well in practice.
I have cheaper adaptors that work perfectly well, so I do NOT recommend this one!