My feeling is that if the concerns which many people are expressing still exist after Ancestry's program emerges from the beta phase, then they will be serious ones. Until that time, I'm willing to give Ancestry the benefit of the doubt.
My father tested, for genealogical purposes, through 23andMe. While I am very thankful for his raw data and for 'segment comparison tools,' there have been many more relevant matches for me in the week since I received my results from Ancestry, than has been the case for my dad at 23andMe and at GedMatch ... in many months. Every day there is much work to be done here, on our family tree, while elsewhere, there has been mostly - waiting. Fewer of the folks who test at 23andMe have trees researched and posted which can, at least, be used as springboards.
The refrain on the message boards at 23andMe is, 'Send your raw data to Dr. Doug McDonald.' Dr. McDonald found my dad to be a purebred - 100% English - and his findings, and the way in which they are presented, concern me. You were correct in referring to him previously as a guru. If a non-Nordic segment shows up in your results then, by all means, turn to him so that he can divine if it is "real" or not.
There is a member at 23andMe by the name of Spence (Oregon 1841), who kindly delves far deeper into one's raw data, using an advanced mode of the 'World 9' oracle. His findings, for my dad, provided a more detailed and accurate portrait of Dad's ancestry than we have received elsewhere. They coincided with Dad's paper-trail genealogy, and also suggested distant ancestry, which we have not documented yet (if this is, indeed, more than 'noise').
I do hope that Ancestry develops a better system for interpreting genetic ethnicity than what it is presenting at present. I can see, for example, that both my dad and I still carry DNA from German ancestors who settled in Virginia during the early eighteenth century. I show no Central European ancestry here, however. Contrarily, neither I nor the distant cousins (whose trees I have perused) with whom I have been matched by Ancestry, and who also have been assigned Southern European descent, appear to have roots in that region during genealogical time. If 'Southern European' represents the legacy of Mediterranean Farmers during prehistoric days, it seems strange that their input would override the contribution of forebears who lived only a couple of centuries ago.