I think ultimately a problem arises when trying to delve too deeply into a magic system, to explain its every quirk and function. I loved reading Robert Jordan's Wheel of TIme series and his 'magic' system is incredibly complex - one of the most complex I've ever seen. But he could often get very bogged down in explaining away every little facet of every little 'spell'. It could become tiring.

Brandon Sanderson suffers from similar problems, I think. His magic systems are incredibly unique, unlike any I've ever seen, but he spends a lot of time explaining. Every. Single. Process. He's a brilliant writer, and his Way of Kings series is one of the best fantasy series I have ever read. But even from his opening in the first book, it is clear that he suffers from the same trap that I think all fantasy writers fall into: we want to show the reader every little detail that we can. We get lost in the myth of our world-building, and sometimes that can really get in the way of keeping the narrative going forth.

I don't really mind 'magic' or any of its equivalents not necessarily having a proper source that 'makes sense'. I've been reading a lot of myths and a lot of folklore lately and to be honest it's really refreshing to just have the stories take the existence of magic and of the supernatural as just as a matter-of-fact reality of life.

Edit: I forgot to mention. Brandon Sanderson also has a fantastic rule-set to bear in mind when developing a magic system