I suspect many people's iOS Contacts have been trashed by iCloud, but only a small percentage realize how bad things are. It may explain some of the mysterious data use and battery life problems some see.
I can't prove this of course; I'm not about to study a random sample of 100 devices. I can only go by my own disastrous experience and Emily's recent malfunction.
In my case the bug had to do with a change between MobileMe and iCloud in the handling of line endings resulting in large numbers of 'Groups' metastasizing between my devices. It was very hard to diagnose and fix that bug; since then things have seemed to go well with my use.
Emily's malfunctions seemed simple at first. I noticed she had both addresses on her device and in iCloud. I didn't expect that, but I figured it would be easy to clean up. . Then I discovered about 3,500 entries in Contacts that showed up only as 'No Name'. This isn't a new problem and I don't know how long Emily had it. She never noticed, I suspect most non-geeks ignore that sort of thing. I wonder if it explains some battery life issues she's had, and even Spotlight and disk access issues on some of our older machines as these malformed entries propagated. (As noted below, during the removal process the OS X Contacts app used 25 GB of virtual memory and locked up my 27" iMac.)
Removing these malformed null-value entries was much harder than expected. Something about processing the deletes using Mountain Lion Contacts.app consumed vast amounts of virtual memory -- almost completely paralyzing my 2 yo iMac. (More about how I got control in a later post.)