The accuracy of this geolocalization depends very much on peering agreements.
I don't know about the UD bit this will not be very accurate within the EU.
As an example:
In Hungary, there's pretty much only one peering hub (bix) and there's only one Cloudflare datacenter. You've already geolocated me better than this hack just by knowing my language or phone prefix.
When I am traveling, i most likely use my mobile data. That data is tunneled to my mobile provider, exiting to the public internet at exactly the same server.
In my case, Cloudflare will identify me as BUD even when i'm roaming at a different country.
This behavior is very typical for the EU, because the telco landscape is fairly fragmented, and each company typically have only one, or at most 2 peering locations.
This may be different within the US where the distances are bigger, and latencies matter more, so there is more incentive to peer locally.
I don't know about the UD bit this will not be very accurate within the EU.
As an example: In Hungary, there's pretty much only one peering hub (bix) and there's only one Cloudflare datacenter. You've already geolocated me better than this hack just by knowing my language or phone prefix.