Additionally to the comprehensive comment above, the lack of a DNA match can be revealing.

I have a dead end in my tree of someone (GP), born in around 1760. His surname is uncommon and he lived around the Portsmouth/Gosport area of Hampshire. I have a DNA match to one of his grand-daughters so my line to him is confirmed.

There is a large cluster of inter-related P's in a village near Portsmouth in the 18thC who were potential relatives. I can now discount them as being connected to my family because I've learnt that two people who provably descend from them have their DNA in the Ancestry pool. They have not appeared in the list of matches.

So no DNA matches can be as significant as actual matches. Its the Holmes 'Dog that didn't bark' factor.