Overbuild is a hot topic and to be frank I've seen some cabinets go in that make little sense, but as the gap funding is adjusted to take account of overbuild, a project is contributing less for those ones.
To do a proper analysis would take a lot of people a lot of time, particularly as to get to the bottom of it, rather than 'opinion' one would need access to historical roll-out data from both Openreach and the alt-net and also co-operation from BDUK projects to review the OMR data they received.
Cost versus benefit starts to look dodgy. Certainly some overbuilds where people complain that BT turned up mob handed are areas where a vague plan has existed for some time before the mob turned up.