One thing that goes beyond kit or ARF design with SIG is the selection of wood. I am no expert, but there are different grades of balsa, light to heavy. And SIG seems to work out the details all the way down to whether a part of the kit should have soft/light balsa or hard/heavy balsa. You won't get that in the cheaper brands.

Aside from kits and ARFs, SIG makes/sells a lot of quality hardware, tools, glues, paints/coatings (dope), castor oil, etc... that I use even if I am assembling an ARF. That part of SIG I would miss more than the ARFs/kits and is not replaceable by other brands, IMHO.