It is often said that the ethnicity estimates are just a bit of fun, etc., you shouldnt rely on them. However I do feel that there is without doubt good science behind them, as my experience is that they do reflect what I know from family history, and my genealogical investigations.
I had my DNA tested by ancestry, and have uploaded it to at least( what a memory!), 4 other organisations, the usual suspects, MH/FTDNA/Gedmatch/Living, and possibly one other.
The first thing to say is that my test was carried out about 2 years ago, and with Ancestry/MH/FTDNA there have been at least 1 update, in the case of Ancestry, 2 or 3 I think.
However, they all come to roughly the same conclusion, I am 75-85 % and even more Irish, the majority of the rest is north Ireland/South west Scotland. There is a little splash of north west Scotland, and possibly a few % (2/4/or<1) which is possibly just white noise in the testing.
Some went into minimal detail - FTDNA said 96% British isles, and 4 * West central Europe, while Ancestry went into by far the most detail., indicating specific areas of Ireland where ancestors likely came from.
The most impressive details was that they indicated Connaught/Galway, and north Connemara specifically. Now my Great Grandmother Margaret Burke came from near Clifden in the wildest, west of Ireland. She came latterly for my family (1870's), and on her own, while most of my other ancestors came from the counties making up Ulster, especially the coastal counties of Donegal, Derry & Antrim. Those who came to Scotland tended to travel across the North Channel to Scotland from the northern counties, while those from counties further south went to Wales and England - and of course to the West!
As a result It may be that Margaret's contribution to my DNA mix stood out and was easier to identify. However they also pin-pointed Donegal, from whence many came to the central belt of Scotland. But I assume that when the test was being run, they did not know where the test donor came from.
So I definitely think there is something in it, and they can in some cases pin-point smallish areas of origin, especially if there has not been that much mixing up of genetic material with that of other areas. Ancestry indicated the Innishowen Peninsula, that part of Donegal closest to Scotland, and that would be where I might start my searching for those branches that I have no idea of origin - like my Duffys - my maternal paternal line, who have been is Scotland since before 1826.
Another point to bear in mind for my specific ethnicity estimates is that people were moving back and forward across the North Channel out of necessity on their own and in groups long before the 'plantation ' times, Dalriada was a western costal kingdom uniting ireland and Scotland, before Scotland was born. The Vikings came from the north east, and ruled in Orkney/Shetlands, the Northern Isles, the Western Isles and Ireland, so our ancient ( beyond the bounds of an autosomal test is what I mean here), ancestry will have been truely mixed up.
On the pont of the break down of Scottish ethnicity, in my case at least, the areas were given as south west/north west/west central scotland, nothing more distinct. However I suspect if you came from a distinct part of Scotland away from the the central belt, with much less genetic mixing, in a way like Connemara, then I imagine the genetic fingerprint for there will be more distinct and identifiable. However perhaps they (Ancestry) don't have the volume of samples to identify that. Plenty emigrated from Scotland to the USA & Canada, but not in the same volumes as the Irish went to USA & Canada.
Jane