You gotta be careful with ND's when shooting color film. Most are not really color neutral. Tiffen and other less expensive brands tend to be slightly greenish. Heliopan is a better choice. Also, they're not always the exact density as labeled. I never trust that until I've made an actual densitometer reading through the specific filter in question.
But I've never found a need for ND's in architectural shooting, except for grad "center filters" on extra wide angle lenses, which are a different category. Not even in landscape shooting. So your whole question is a bit of a mystery to me.
These seem more like a product trend looking for an application, rather than a problem looking for a solution. I've been shooting large format in both color and black and white for the past 45 yrs, and have never once carried an ND filter along. Are you perhaps trying to slow exposures down for sake of blurred running water etc? In black and white film usage, ordinary contrast filters can do that.