Well, when I think of middle schoolers I think of my sister who is in middle school. There are times when I think that she is fully incapable of learning, but weeks later she will surprise me with some tidbit of information that she had retained. The opportunities for middle schoolers I think are that they are very open and receptive to modeling. Middle school is a time where kids look to idols and mentors and it is a great time to be a positive mentor and introduce strong Christian figures and heroes that they can look to i.e. St. Francis, Martin Luther, Shaine Claiborne. Especially for my sister it helps her to see real people that live a life like Jesus in the here and now. For the recreational aspect middle schoolers love to play games and have fun. As far as limitations, I think that middle schoolers can dig in deep to the word, but their attention span and thinking capacities are similar to that of a small rabbit especially if social media is not utilized. Also for recreation sometimes kids don't want to cross the middle school cliques and hang out with the other kids.
I hope this question isn't confusing. The more experience you have had with middle schoolers then your answers will probably be better. My experience is limited.