Many of those require exceptional Talents or skills, not to mention that to go further more than intuitively (as Nynaeve does), you need to have a proper medical education. What Nynaeve does remains crude in comparison to Semirhage, and much of what she accomplished or discovered is the result of daring to take risks where most would have declined to even try (AS Healers think it's better not to try something, or to let a situation stand, than to end up damaging or killing a patient. They take some risks only in life threatening situations, where the patient will die if they do nothing, and even then it seems some of them would prefer to admit they can do nothing than try and be seen failing. A lot of Yellow would have let Dobraine die rather than try something as Samitsu did).

Semirhage's so good because she's got an advanced knowledge of biology, pathology, anatomy and so on, resulting from the best formal education in the AOL. She's able to solve the problems specifically without touching anything else because she can diagnose the problems in the first place, and understand how the human body works, even in the finer details (she also developped her personal skills as far as she did because she was a sadist who never let medical ethics get in the way of experimentation. She's like a surgeon who wouldn't mind killing several patients or use very experimental procedures on unwilling patients in order to develop her skills). People like Nynaeve don't even understand the basics - she probably only have the most vague notions of internal functions.

Several highly regarded Healers in the Yellow (like Romanda herself) are not very good at Five Power healing, despite having a strong talent at the simple Healing. Don't forget Jordan said the Restorers in the AOL were the highest regarded and most valued channelers. They wouldn't have enjoyed that status as a group unless the Talents involved were exceptional and quite beyond the reach of the average AS.

It's no mystery how the Restoring arts vanished in the Breaking. First, we know even in the AOL it was an universe of specialists (much like our medicine). The whole world was turned into "the frontline". Put everyone with medical knowledge in situations where they must struggle to survive day to day, where all medical schools and medical books gradually disappear, where the means to find, recruit and train everyone with the potential are gone, and see how much of "modern medicine" survive three centuries worth of this. Long before the breaking was over, very long, there wouldn't be many fully trained Restorer, and many specialities would have lost all the Restorers who had the theorical and practical knowledge to perform them and teach them. Even the first generation of new AS during the Breaking would have an incomplete formation, the more sophisticated and requiring long studies the arts, the more incomplete the formation. It's also fairly likely the situation didn't let many channelers have the luxury of long training, before they must split from their teachers or lose them. A few decades later, there would be even less fully trained Restorers alive to train new ones, less and less material to train them with (medical books, equipments etc.) and it's people with incomplete formations who would teach, and so on and so on, until what remained of the healing arts were but a shadow of what they used to be. It's not so surprising that what emerged/was developped first as battlefield healing in the War of Shadow, something far less effective than the Restoring arts but that could be performed by many more individuals and without medical knowledge eventually became the norm of Healing. It's even likely full Restorers preferred to spend the little time and opportunities they had left to teach to teach that to 100 women than attempting to train 2-3 apprentices in the full Restoring arts. They would have saved many more people training the many in the simple stuff, not the few in the more sophisticated arts. It would be made worse by the fact it would have taken a long time before Restorers fully realized this was not a temporary situation and things wouldn't be back to normal for the AS organization any time soon (this is true for most talents, actually). For a long time, AS would have approached training believing they had to make it extremely expeditive temporarily until they managed to stabilized the situation enough to return to the normal ways of training (this suggests further losses of OP knowledge that had survived the Breaking must have vanished during the Trolloc Wars, when the Tower also had to speed train sisters and send them out of the Tower and into the war. It's very likely after the Wars, they were nowhere as many "fully trained", pre-war AS left alive to teach everything again than the Tower hoped for, and that many of the rarer stuff, requiring special skills, either preserved since the AOL or rediscovered since, have vanished for the Tower curriculum then).

In the case of Restoring, there's another factor involved: we know with gender-based specialities and a lot of mix-gender work involved (again, true for a great deal of AOL advanced channelling). The Breaking wasn't exactly a period where female AS had a lot of time to sit down and think how to change their ways so what they've always been trained to do in mix-gender situations, or to let saidin users deal with, or do a part of the work, could be done by women alone. The most inventive and proactive channelers would have found ways to deal with that, but a great deal of people, probably most, would have lost a great deal of their confidence and means in that situation. A lot of people are extremely conservative in their thinking. Rather than try to find ways to do something with saidar that traditionally were done with saidin and so little practiced by saidar users, they would just give up and consider "this can no longer be done". I think it's very likely why the art of making angreal vanished. It seems extremely logical that they either involved mix-gender work, or a vital part of them had to be made by the other gender (it seems a logical conclusion from the way a saidar conduit made by Nynaeve for Siuan/Leane restored a minimal part of their strength, while a similar procedure made with saidin could restore a woman's full strength. And vice-versa for a gentled man). Women would have stopped making angreal for men (meaning the practical knowledge to do it would likely have stopped being taught at all), of course, and men were eventually exterminated. The flaws in Callandor, rushed during the WOS - possibly to provide LTT with a powerful tool after the Fateful Concord excluded women from his plans, suggests it's not easy to manufacture san'angreal and they are rather high tech tools (though minor angreal are possibly far easier to make, and appear so personal at time in design that it seems they were made to order by individuals).