Latest Sherlock: I think we as viewers are being let in on something the characters don't know yet. If so, I hope they catch up soon - not good to let your audience get too far ahead.
Basically I think Joan and Sherlock are making a fundamental mistake in Joan's training. Not that learning some of the skills she is learning is a mistake. But the basic intent seems to be to turn her into a 2nd Sherlock. And on the one hand that is not possible, because some of the what Sherlock can do can't be taught. And on the other hand it is the wrong goal in any case. Sherlock already is Sherlock. He does not need a mental twin. Joan has been the most help to him through her ability to complement him, to do stuff he can't. She has empathy, social skills, medical knowledge, and an ability to spot breaks in patterns that reveals stuff Sherlock's own brand of deduction does not. They work better together than separately.
Now maybe this is just head canon, and I'm reading something the writers don't intend. But I suspect we are intended to see this, and the characters will realize and explicitly say it in an episode or so. When Joan says "I can't do this" she is both right and wrong. She can't be another Sherlock, but she can go on being an awesome Joan who can do all sorts of things Sherlock can't do, that may be more important than what Sherlock does. At minimum they let Sherlock do what he does far better than if she was not there. At that last sentence IS canon.