The documents (birth and death certificates, marriage licenses, US Census) they point you towards are as accurate as written. Ancestry does not make up the document, they just help you find them.

Family trees made as posted can be full of short-cuts and outright errors, though. Just because someone has listed a relationship does not make it accurate, so do your own research before you accept that you have a long-lost Uncle Albert. One of my family trees listed my grandfather and 5 brothers, all with the same given name! The person who made this tree likely only knew about grandpa, and heard he had 5 brothers, so used grandpa's name in the program as a 'place holder' and then published this tree online.

Important Genealogy Rule - Verify all relationships with documents.

Important Genealogy Rule - Be sure to note and then cite your source(s) for relationships and other family data!