I tested a relative with 23andme so I could see who of relevance was in the database and learn how to use it properly in case I ever have a client with it.
There is a kind of a tree that they put in for you based on your matches, but it's all over the place. They've placed a confirmed and documented match (3rd cousin) on the wrong grandparent's line. Of course, it is possible this match also matches the other grandparent, but seems unlikely to be right. There's no way to correct it either (like wrong potential ancestors on Ancestry's Thrulines).
I've also found a lot of people don't respond and don't fill out the section with where their ancestors came from.
Furthermore, I've had some of the most unhelpful replies ever. Response to polite with some detail query about likely connection "no, I don't think so."
Similar to people testing with Ancestry for the ethnicity reports, and not logging in again.