Knacks aren't really good enough, in general traits are far better, this is good, but I have two rules that modify this a bit.
a knacks of 3 or 4 allows the char to make one raise after the roll is made. Ie. if the char has made a good roll, he can take a raise afterwards.
A knack of 5 or 6 allow two raises to be made after the roll
Instead of any decent NPC doctor being an utter genius, the NPC-only advantage Specialist allows the NPC to have one speciality skill, any roll with knacks in that skill are made (Trait+knack)K(knack).