The last time I tried it the results weren't great. That in itself isn't a problem but most of them were plagiarized from other designs (from the web). It wasn't easy to track all of them down, so it is probably reasonable to assume that a lot of the designs are rip-offs, and most people don't notice.

99designs refunded anything that was paid, and apologised, and it felt like something that is so common they have a well oiled process for it.

Given the risk of this, and that it is up to you to find if it is a rip-off - I wouldn't use it ever for anything that may be seen publicly (I expect you as the "owner" would be responsible).

(this is ignoring designer objections, which are another topic).