Based on my limited sampling of Bourbon and Scotch coupled with knowledge that has been passed on to me from individuals whose Bourbon and Scotch collection is vast, yes I would say the Bourbon spectrum is much tighter.But would it be much wider than the spectrum from a straight Rye Whiskey to a straight Wheat Whiskey (realizing that wheat whiskey is a new phenomenum)?If you took all the flavor varieties of Scotch and lined them up (I don't mean brands) they would be far wider than all the flavor varieties of Bourbon.
That is not to say Bourbon is less interesting or bourbons all taste the same, it just seems to me that Bourbon (Rye through Wheat)has a closer range of flavors than Scotch. I think despite its tighter range of flavors, Bourbon has some amazing and unique flavor profiles. There is no reason to feel Bourbon is inferior to Scotch.
Here's a few questions for those with greater knowledge on Scotch and Bourbon:
Is there something about corn that leaves a greater flavor characteristic in Bourbon than malted barley does in Scotch?
Or is it the greater number of distilleries and various regions of Scotch, which accounts for the great range of flavors found in Scotch?
If Bourbon's popularity continues to grow and more and more distilleries open, in years to come would you expect the flavor profile of Bourbon to widen?