What a nothing of an episode.
I think I know what "Apple Family Reunion" is going for. It's trying to have some sort of warm feeling, reflecting on the values of family. But it forgets to actually have any interesting elements-or much in the way of a plot, really-in the process. It's not intolerable, or dull, or even that bad, it's just a mediocre little stepping stone to the next good episode.
The actual conflict of the episode doesn't really become apparent until more than half way through the episode, and by that point it's almost at the climax. The idea was that Applejack is trying too hard to make the reunion great, but so little happens in the episode that it's not readily apparent this is supposed to be a problem. Without a compelling conflict, the episode falls flat, but it could have still been good with great comedy.
Unfortunately, that's lacking here as well. There's a few mildly funny moments, but the episode is largely lacking in the laughs department. Sure, there's a lot of events, but they're mostly warm, amiable things which don't leave a strong impact. Perhaps this is partially due to the utterly forgettable score, which not only fails to convey even the slightest feeling of urgency, it causes the things happening on-screen to fall flat.
It's not awful, of course. There's a fair amount of nice visual elements, particularly the rainbow of fruit bats, and the feeling of warmth is at least there. Some small moments are nice on their own, but without anything stronger to support them, they leave less of an impact than they would otherwise. It's unfortunate, but the episode is ultimately less than the sum of its (fairly decent) parts.
At the very least, the song "Raise This Barn" is catchy. It's a fun little tune, with energetic fiddles and a sing-along chorus. In particular, at the end it slows down and becomes a sort of Apple Family Choir, and the harmonies in this part are wonderful. The song is probably the best part of the episode.
"Apple Family Reunion" is one of the weakest parts of S3, and while there isn't much to pick from, the episode is just so ho-hum. It has quite a few strong elements, but the skeleton of the episode isn't strong enough to support it. Of all the S3 episodes, this one's by far the most forgettable.