I recommend people do the family tree DNA test before doing ancestry or 23 and me. The latter two have language in their licensing agreement to reuse your DNA for testing purposes. Family tree DNA doesn't.
However, ancestry has the biggest database.
They all three have language that states they will turn over DNA when requested by law-enforcement. So, your childhood indiscretions may come back to haunt you.<img src=http://www.ar15.com/images/smilies/icon_smile_big.gif border=0 align=middle>
The Y DNA that males inherit from their fathers mutates rather slowly. It's possible to find relatives back before recorded ancestery in England. The trick is proving it. DNA testing can also reveal that great great great grandma was not honest about who she invited into her bed. So that 100% pure Welsh paper trail may turn out to be less than perfectly accurate.
All three have an origins testing reference sample, which should improve as time goes forward. But our DNA, unlike paperwork, can't be forged, burned in a courthouse fire, or given away to cousins who probably lose it. Genetic DNA testing by itself is fun, but works best in tandem with paper trails documenting births marriages and deaths.
And for the super paranoid, the lizard queen says you need to report to your body suit suppliers in the next 24 hours for updates. They learned a lot from extended usage by Hillary. <img src=http://www.ar15.com/images/smilies/2016/blink.gif border=0 align=middle><img src=http://www.ar15.com/images/smilies/smiley_bucktooth.gif border=0 align=middle>