One thing I've not seen any other Reddit* or Lemmy app do that Sync does, is preserve the "position" of comments. If you enter a thread, scroll down half way, then go back, enter a different thread, leave that one, and come back to the previous thread - you'll still be at the same spot.

This is nice for me when I accidentally swipe the wrong way and end up on the previous page, or something happens to catch my mind just as I'm going back a page.

Specifically, I like a lot of things about Connect but I've found that there's no decent separation between different comments/posts. Aside from Sync though, I really like Summit.

*Apollo somewhat did this, IIRC you had to use the swipe forward gesture in order to return to the previous position. But if you loaded another thread (or even just did a regular click of the same thread), that didn't work. I also haven't been on iOS for a while now so I could be out of date on that.