The iOS 8.2 update, just released, claims to fix some problems with iTunes syncing. We’ll have to see if this resolves some of the most egregious sync issues. Feel free to post a comment if this fixes things for you (or not).
In my first few tries, removing music, and then syncing new music to my iPhone 5s, the sync started fairly quickly, and seemed to sync all the music I was adding. Syncs are clearly much faster than before. Let’s hope this lasts.