I think it depends on the cabinetry and the experience of the GC. If the GC knows the brand or the work of the custom cabinetmaker, then it might not be a big deal. If not, however, there could be problems. The GC on my job, who is also a master carpenter, had never worked with Plain and Fancy before. Although the installation went ok, it could have gone faster had this not been his first time dealing with P&F. Honestly, I can't imagine anyone being better qualified to install your cabinetry than the cabinet maker's own team. At the same time, I can also understand if your GC is not happy with that arrangement because it will cut him/her out of some nice fees. :-)