From what I understand, there are no stronger and weaker skills. Like @prometheus mentioned elsewhere, there's no such thing as a fireball and major fireball. I believe the spotlight about the knowledge system talks about the way unlocking works. https://fracturedmmo.com/feature-spotlight-3-knowledge-system/.
I don't think there's much of a point in investing in only one tree. If you want to be a pyromancer, you'll take your fireball from Invocation, your AOE fire attack from Conjuration, your fire shield (assuming there is one) from Abjuration, etc.