See, the thing is - I use sync across multiple Apple devices. I have had zero problems with duplicated or missing entries, and it's one of the absolute killer features of the ecosystem for me.
It's hard to get a real impression of what problems may or may not exist, unfortunately.
That was just the first link from a Google search.
Internet forums are full of other users complaining of the same problem.
Even if it only affects 0.1% of users, multiply that by the tens of millions, and that's a large number of people having a horrible experience.
It works great for you, until you're in the 0.1% then it's a bitch.
Edit: and it's not just music, it's a mulit-year history of botching up sync across multiple different technologies - some of which I've experienced first hand, some of which I've had friends complain of. Either way, Apple have zero credibility with me and syncing.
It's hard to get a real impression of what problems may or may not exist, unfortunately.