Our friend Daniel Jalkut ran into some pretty serious issues when trying to clean up his Contacts list:
After painstakingly merging cards in this manner for an hour or so (!), I had stopped paying close attention to whether conflicting data was being persisted well or not. At one point I stumbled upon the realization that I lacked the phone number for a contact whom I had sent an SMS message just within the past week. Other contacts were missing key data, too. An outdated email address here, a missing mailing address there. Whoops! Abort mission! Time to recover from that backup file.
Spoiler: his local backup was incomplete, and he had to eventually go to iCloud to recover his original—if still messy—data.
I ran into a similar problem earlier this year, when trying to remove duplicates in my Contacts database. The app decided to merge cards for people who lived at the same physical address—i.e. my mom and dad, my uncle and aunt, and my cousin and his wife—into a single card. I think I’m still untangling parts of that. So be very careful with those features.