I feel like Amy won't hestitate to make episodes about mature themes, this episode included.
The dilemma of production conflicting creativity is not something I'm a stranger too. While I myself haven't been subject to this, many friends of mine have, and this episode touched them on a personal level, which is a HUGE goal to accomplish as a story writer. Very nice moral, very nice song, and very funny. (Especially the fat pony at the end)
Rules of Rarity is a very unique song. The transition from happy to sad is beautifully executed, and adds a very nice depth to the song.
The chords at 1:17 are honestly the best part of the song, and leaning to become my favourite part of any song in the show.
Overall this was a very nice episode. The moral was very mature and relatable, the song was one of the best so far, and SO MANY NEW MARES! Masterpiece of an episode, Definitely in my top 5!