YouTube captioning tends to range from bad to horrible. TED talks are probably hand-transcribed, as I remember them as being higher quality.
Yes, and I mentioned YouTube specifically because it's representative of the best machine transcription (which this service is) can offer.
TED talks are indeed transcribed by professionals, and so the quality is a magnitude better than what this service can provide. TEDx talks are transcribed by volunteers, so their quality is more variable.[1]