Yeah, prior to DNA testing, documentation is all we had, and even when there's a marriage record, not everyone is a fanatic about that. Also, I've found that other people doing the same lines find stuff I'd never heard of before (one of the sibs was born to the second wife, who only lasted a year before my grandcestor was widowed a second time), and there's also even less savory events that didn't get recorded. One of my cousins ran into a foundling ancestor who was a young child sitting crying on a late middle ages battlefield, and someone from the winning side scooped him up and raised him as his own.
My aunt was a genealogist, professionally and for her DAR chapter. She was a snob, but also very honest about her work. People would get mad if she found that their ‘true’ ancestor, according to actual records in old churches, graveyards, etc., was some common menial laborer instead of the fancy person they had assumed...