We have just done a block in Ponteland and will potentially do more on other sites but it depends on the development really.I just don't think there's much demand for them. Outside of London & big city centres, as they're very difficult to sell. Round here, usually end up being rental properties, and if you're buying a new build you don't want a load of renters on the estate. So having flats on the estate could put off some buyers & could even devalue other properties.
If you need an amount of affordable homes on a site they can be a good way to achieve it if you do them properly.
We did some in Alnwick and the holiday let money the owners can pull in is nuts, 700 a week in a lot of cases.