There is plenty of info about all the problems with SKShapeNode in iOS 7.
Memory Leak in Sprite Kit Application
http://sartak.org/2014/03/skshapenode-you-are-dead-to-me.html
Have all these issues actually been resolved in iOS 8? There aren't any posts later than August 2014 describing problems, but I wanted to make sure before I committed to SKShapeNode.
Update:
According to LearnCocos2D you probably shouldn't use SKShapeNode for serious games. However, from inferring from what I've been reading, and from my (brief) experience, SKShapeNode won't cause any memory headaches during casual use. I'm inclined to assume this is true, unless anyone has information to indicate otherwise?