Large scale NAT is one reason that would fail. Some mobile carriers have most of their customers behind a handful of IPs. Wikipedia has dealt with this a lot, and has a page explaining why conflating an IP address to a person isn't usually the way to go.
Not really. Problems arise when you have many people behind the same IP. You can hand-wave this problem away for family units behind a router, but you'll soon find out there are entire institutions (schools, office buildings with lots of unrelated companies in them, etc) that will share an IP.
Yes, but with a significant false positive rate due to IP address sharing, and you can get more articles by switching IP’s (unless someone else exhausted the new IP before you got it).
https://en.wikipedia.org/wiki/Wikipedia:IP_addresses_are_not...