I'd say in England it's pretty okay back to the start of C of E registers, I'd one line shot straight back in that direction, but even so, not perfect,( in one generation there's an "either/or" so you can't even count that,) but those fanciful ones with oodles of nobility / royalty, and "My granda was the rightful heir to the Scottish throne..." at least amuse those who enjoy them.
Most can't honestly be sure long before 1800 unless there's both unusual surname and first names, and firm geographical location - and loads of documentation from independent sources, not "My aunt alice's grandma always said..." - yes?