The accuracy of vent sexing falls on many factors...
A) *The most important - who's doing the sexing? How experienced are they with vent sexing that particular genera/species? I have even seen many people incorrectly molt sex by mistaking male accessory organs for spermathecae.
B) The size of the specimen, It obviously gets easier and easier to vent sex as they grow.
C) The quality of the photo(s). Best to vent sex in person as lighting and angles can be manipulated.
D) Some genera/species are easier than others.
Truth be told, I haven't molt sexed any of my Avicularia in a long time. They are by comparison much easier to vent sex than most genera. Once I got the hang of it and confirming with a molt, I haven't been wrong yet. I have seen so many Avic bits, I'll be certain by 2-3". I still confirm by molt with any other genera I'm not as experienced with.