i sync with my iphone 5c just fine... and i have a library of over 5k. only about 300 of those songs are itunes purchased. i am able to get any of the music that i want synced onto my iphone without problems and im using a windows 7 computer.
my point is, when it works its awesome. and its designed to work. 99% of issues are due to user error or user not knowing how to fix problems that come up. technology comes with problems. the more complicated the technology the more problems that can pop up. the only way to avoid those problems are to not use technology. barring living under a rock, it is advisable to learn how to deal with those problems when they pop up.
and thats why there are 62 pages of conversation over this issue. "not able to sync" can be caused by a giant range of different causes.
my advise to most people that dont want to go back to their cd player:
- delete ur library out of itunes. when it gives u the option to move files to recycle bin or keep on computer, choose to keep. then use the option that pops up in ur empty itunes library to "scan for media". this will fix any issues that are caused by broken links with ur music library files.
- uninstall itunes and components and reinstall. the proper method for this can be found easily by searching the apple website. follow suggested procedures! this will fix software related issues that are in the program files of itunes.
- restore iphone. also something u can easily find out how to do using apple website search. just make sure to serach that phrase exactly. u can do a back up and then restore back up onto the phone after the restore iphone phase if u want to keep ur information but be warned, any problem ur trying to fix with this solution may travel with the back up. i would advise setting the phone up as new instead. this solution will fix any software related problems u may have on the phone itself. that goes for any apple device, not just iphones.
if none of those solutions fixed ur issue then the problem is with the computer u are using, or the mobile device's hardware (mobile device hardware being the cause of this issue is extremely rare).