Hi, not a professional historian, but I am a genealogist.

Ancestry is effectively half family tree builder, half search engine of records. These records are collected from third party sources, such as the governments or organisations that produced or held onto them. As such the information Ancestry has is only as accurate as the original source. This means names can be spelt multiple ways due to illiteracy, or people can get confused due to having similar names, ages, and being from the same place. However this has less to do with the sources themselves and more to do with a lack of sources. This is something anyone who does family history, or history in general, will probably have to deal with.

Speaking for myself, I'm a white Australian so the sources I've used from Ancestry are very accurate, barring any mistakes I've accidentally made. The Australian government right from the colonial days kept pretty good records, and many vital things such as convict and immigration papers are digitised and transcribed on the site.

Theoretically, Who Do You Think You Are, is just a televised extension of someone using the Ancestry service, with a healthy travel budget not everyone can afford. Although from what I've seen in the show, the sources used from outside Ancestry are generally from other reputable places (e.g. an official list of Holocaust victims in the Stephen Fry episode) or official church books from real British (or otherwise) parishes.

Whether or not things are sensationalised is more up to your personal opinion though. In some episodes they find ancestors who genuinely did interesting things. But on the flip side I remember one episode where they found out the guest was a descendant of Charlemange, which is kind of cheating because so is practically every white western European. And then again in others they just trace it back to an ancestor who lived a common life, and conjecture on what their life may have been like as a peasant during the industrial revolution. Safe to say by modern standards, the answer is inevitably, not great. So I'd call that a bit sensationalist, given just how common low standards of living were back then, it seems a bit cheap to spin it into a sob story. But it all depends on how you look at it.

Hope that helped.