This seems to have the same issue as Pirsch[0] in that (i) a mobile user will have multiple IPs during a day as they roam between wifi networks and mobile networks - which will incorrectly over-report unique visitors and (ii) NAT gateways with similar devices behind (a home with 4 iPhones, a workplace with 5000 identically imaged desktops, all similar devices at a starbucks, an entire lab of computers at a school) will incorrectly under-report unique users.
An IPv4 address is an unreliable way to count "a visitor". At best it represents "at least one network (~building) of 1-n visitors". You could argue 0-n visitors (some sort of automation/crawler). It could also represent more than one network (a temporary IP obtained for less than a day, a IP obtained for a day that changes during your definition of a day).
User-agent reduces the scope, but doesn't eliminate (between recommended evergreen updates and identically imaged/uncustomized use of devices).
I mostly agree with your comment. It's really hard to solve these issues without violating the visitors privacy and saving state on the client device. I think the approach is "good enough" if you want to know how well your website performs or to detect trends, but you will definitely need more invasive methods to gain more insight.
An IPv4 address is an unreliable way to count "a visitor". At best it represents "at least one network (~building) of 1-n visitors". You could argue 0-n visitors (some sort of automation/crawler). It could also represent more than one network (a temporary IP obtained for less than a day, a IP obtained for a day that changes during your definition of a day).
User-agent reduces the scope, but doesn't eliminate (between recommended evergreen updates and identically imaged/uncustomized use of devices).
[0]: https://news.ycombinator.com/item?id=23668212