One of the cons is that it takes a LOT of practice even with instant feedback.
I saw on a documentary that some professional training programs don't provide any pictures or instruction other than having the students watch pros for a short time. Then they just do it. They do many thousands a day and eventually, months later, they start getting it right. Until they get it right often enough, the chicks get sexed again by a pro. The training program had tried other ways and went back to this as it was the fastest, most cost effective way. It was in Japan, I think. I don't know if training is done differently in other places.
And, yes, it is very easy to hurt the chicks. That is why vent sexed bantams are not available. The pros could tell the difference but even they can't do it without hurting some of them because they are that much more delicate. This part is from sources I don't remember so am not sure how reliable they are other than I considered at least some to be reliable.