One of the guys at my model train club said of Model Railroader "Dream it, Plan It, Buy it".
The thing is if you use kits for everything, a modeler can look at your railroad and spot this building or that familiar building (the non modeling public won't be as susceptible to this- but we have already impressed them) , and they will see your railroad as a bunch of pieces, rather than as a whole; and at that time whatever effort you have put at building the illusion of a unique miniature world is out the window.
so, while some kits are too good to pass up, kitbash the buildings around them, and scratchbuild some main focal points of a scene, and the scene , where only one or two buildings looks familiar, can have enough integrity to help the viewer suspend disbeleif, and view the scene as a whole.
Bill Nelson (scratchbuilding all the time over in the Logging/mining/industrial section.