Time Machine will regularly (as in once a month or two for my household of Macs, sometimes in as little as a week) complain "Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you." Apple says don't backup to a NAS. Plenty of folks have reported problems.
Getting it set up in the first place [0] keeps the selection of customers significantly smaller than those that are better off just buying a Time Capsule, or even sticking a USB drive onto an Airport Extreme (which I recall seemed to work fine). And Apple did say it's not supported. I wish they'd just fix it. IIRC, it worked reliably pre-Lion. EDIT: or for that matter, why not just bundle up the page I referenced into a nice app bundle with a pretty icon?
A more significant problem is that Synology (which is what I use) is out there saying, "Time Machine: works great with a Synology NAS!". And it does work great...until it doesn't, and last I checked Synology has no docs on how to fix it.
As for Mavericks, I'm pretty sure I've seen the problem on the one machine I have running the dev preview. Or maybe not, maybe it was before I put Mavericks on (I realize that's not terribly helpful information). The problem with saying whether it's better or not is that on my machines it may be a month or more before it rears its head. So it would take me six months or so before I could personally say that it's better for me.
Buy a Time Capsule if you want the Apple experience. It will cost you, but then it's Apple's solution.
Apple Time Machine has never been friendly to NAS other than Time Capsule. You have to do some hackery to get it setup, so the above fix steps are suitable for that crowd.
Thankfully it's easy to fix: http://www.garth.org/archives/2011,08,27,169,fix-time-machin...