It's actually kind of a dilemma. Some people want to see their ethnic background in "genealogical time" to help confirm what they already know (e.g. my mother is Italian), and others want to go back prior to that since traditional genealogy no longer works (how did my "people" get to Italy). As you said, it all depends on the reference populations they sample and what markers they chose to match on. No matter what service you use for autosomal testing you still have the same issue of information getting lost each generation and not being inherited evenly.
To me the bigger issue is even if there was some way to give you a 100% accurate ethnic breakdown at some specific point in time, how does that help you find or confirm individuals in your family tree? Once you get past all the relatively recent intermingling in the U.S., chances are the potential individuals for a specific spot in your tree are of the same ethnicity anyway.
I've seen numerous useful messages here about different techniques people use to process their cousin matches, as well as examples of successes. I've yet to see anyone post a message on how they are using the ethnicity to further their research.