It's a very interesting point that you raise. Obviously, you can't 100% rely on any information that is provided by people. After all, we're all fallible. Personally, I have found that the least reliable information is contained in Irish death certificates. Especially when you're talking about the beginning of the 20th century, when, according to what I can understand from older people, most children didn't know much about their parents i.e. date of birth, date of marriage (if the parents were even married!) or sometimes who their parents' parents were and where they came from. Some people couldn't tell you much about their fathers as many men had to travel the country to make money for the family. But, having said that, the information that we can gather from bmds can be useful to an extent and sometimes very interesting!