why DNA can't always be trusted & the ambiguous info
Part that can be trusted more
First let me start off with the part about DNA that can be trusted the most. BTW they are somewhat related, 2 of them. One is paternity and the other is crime. What makes them related is for the fact that the DNA is recent which will make it the most accurate. When i say recent i mean it doesn't come from lines of people for many years which DNA changes slightly over time not just that its not old.
Confusion, things not said and even fraud
The biggest problem with DNA regardless of what type of test is when checking for ethnicity or ancestry. There are 3 test for this, yDNA or father line with males, mtDNA or mother line with males and females, autosomal which is a cluster of all DNA branches for males and females. What is also overlooked is who was tested to define a original type of DNA. The best example is Native Americans, we all know that most pure blooded Natives died out and are now mixed. So how will we know when getting a definition of who's really Native if this is the problem? Plus not all Amerindians may have the same DNA whether similar or not. Other people have same story and in history also travelled from place to place or left their homeland like 98% of the Israelites, so how to trust a DNA sample on them? Just like other races or group won't have the same DNA even within the same race or group. What people need to understand is that when it is told that a certain group has a specific type of DNA it just means 50% or majority, not all in that group. An example of that are people, mostly male preist, who are Cohanites or descendants of Moses brother Aaron, which not all were tested. From the ones tested it was found about 80% are similar but so what about the 20%, what does the 20% say. The next problem are the racial categories, they are too general and need to be detailed or broken down for better accuracy. Most just have 4: "white/European", black, Amerindian and Asian. European is in quotes because some people can be this only in consideration but may not actually be European nor be pure European as well. sometimes in this group are Arabs who nearly all are mixed, Arab is really just a language and cultural zone. Also some blacks/Africans may be in this group like Hamitic (Berbers, Somali, Ethiopian etc.). Black can mean various Africans, possibly Indian (mostly Dravidian), but what about Melanesians and Australian Aborigines where do they fit? With Asian does this include Central Asians, Indians and all the way east to Polynesian not just mainland East Asian? I've stated the issue with Amerindians before hand but if Amerindians were considered Asian (Turkic) by some then why do they have a separate group? Some historians even say original Natives are only half Asian and a dark race(s) like African or Australoid.
With haplotypes (haplogroups) the average person nor is it stated how different, lets say percentage, the groups are. Reason is that there are even differences within the same haplogroup. Could some haplogroups (given letters) be similar to other different groups and some be very different/unique to others? There are people with similar halplogroups as of between Africans and Europeans but they are still considered their own race and not possibly mixed. There is also one that both some Africans and mostly Indians have. yDNA R is Asian or mongoloid found in Central Asia but some Europeans have it. The only yDNA that is exclusively European is 'i.' There will always be a few odd ones in a group.
With the few black Americans who do DNA ancestry testing, mostly either mtDNA or autosome/autosomal, they may find things don't add up. What is noticed at times is that if they do more than one testing with different companies the results are not the same. One company will give one result the others may give different results. A few stories even find out they are not black or have little black/African DNA even with dark skin. Some also claim some Native ancestry but again what is considered Native, is it coming just from a few tribes? The stats from those who did DNA also say that on average they are 1/8-1/16 white and the ones who had Native blood averaged to around 1/5-1/8. Most say it is at most 1-10 to about 1-20 of blacks have traceable Native ancestry (in consideration). The white ancestry may not be accurate either since some European haplogroups like yDNA are the same as some African and some blacks and/or mixed people are included as "white." since black Americans come from many places within North-West Africa which had Bantus, some Arabs, various Berbers, Moors and even a few Gypsies (both North and West Africa).
In reality all people have different and some have similar (doesn't mean the same) but different DNA whether related or not.