The bottom line is that these companies, even the top-tier ones, are pushing an inexact science as something you can really rely on. Well, you can, but only to a limited extent. They are, in the final analysis, basing their "algorithms" on the population in their databases, which they proclaim to provide enough of a comparison base to new entrants to make their projections. The fact that they change those reports periodically should tell users that what they're getting is likely only a decent guess at where their ancestors came from, not "who you are".
They're exploiting, in an increasingly fluid cultural environment in which a sense of identity used to be clearer, a human craving for knowing who they are, for identity, for something to cling to.
But with the advent of increasing secularism and multiculturalism, for better or worse, things that used to give people their sense of belonging, doesn't any longer, and I really do believe humans are tribal at heart and will go to any lengths available to feel tribal.
23andme, I suspect, does a better job of projection not least because it does mtDNA testing (mitochondrial DNA, which you only inherit from the maternal side), which Andestry does not. They both use autosomal techniques, but 23andme also carried out mtDNA testing.
Therefore, Ancestry's "estimates" are less presciide, and they simply will not nail some level of confidence to the mast in their results, except through those broad ranges, which often looks absurd to laymen.
So, for as much as it's worth, and it's not worth that much, I do think 23andme is better at DNA, while Ancestry is more focussed on broad ethnic ancestry and genealogy.
Lastly, many people misinterpret what those percentages mean. Getting a result from 23andme that shows, say, 12.5% Turkish DNA, does NOT mean you ARE 12.5% Turkish. It only means that in the DNA sample you provided, that was long enough for them to measure and compare with your other strands, 12/5% reflected Turkish origin.
It's a difference that escapes many users of this service, because they want to know what they ARE. No one, anywhere, can tell you with any integrity that YOU ARE this percent of anything, unless all your ancestors for the last 200 years came from County Mayo or something like that. If they tell you that they detected 85% Irish DNA in your test, yes, you likely ARE a huge percentage Irish - only you already knew that, so why pay for it?!
And the reason 23andme can drill down to fractional percentages is because they test mtDNA, so their DNA analysis tools at this point are sharper than Ancestry's.
But hardly infallible. This is still not an exact science.