IMO, knowledge skills in Shadowrun are like knowledge skills in HERO system, or any other one that's open-ended the more vague your skill name (KS: Everything!!) the less useful it becomes when you need specifics. Conversely, a specific skill (KS: Hermiticism) may be totally useless in most circumstances. A KS: Magic Background skill is going to give you general knowledge, yes, but if your knowledge need is for something specific, you're really going to have to rack up the hits.
Also, be careful with specializations. A specialization is generally going to be something only one or two steps down the framework; Magic Background might have as appropriate specializations things like Sorcery, Native American Magic, or Unified Magical Theory, but a specialization on 'alternate magical traditions' may not be particularly appropriate - or, considering all the many things you want to be able to know ("Beliefs. Goals, factions, reagents and the like."), might simply not be available as a specialization.
Alas, I'm still working on going through all the adventures &c. and culling out all the stuff, including as complete a list of in-game examples of knowledge skills as I can manage ...