Both Facebook and Twitter employ a large number of human moderators, and they still regularly make judgment calls that are comically bad - defending stalkers while suspending their victims, suspending users that are the target of coordinated harassment campaigns, etc.
The reality is that when you have an army of people who need to churn through a vast amount of content per day, without the time or authority to look at the full scope and context of user interactions, they will make lots of bad calls. Doubly so if the moderators live half a world a way and have little awareness of the culture and norms of users they're asked to evaluate.
Add this to the fact that the really obvious abusive comments can and are already being moderated by automated systems, it means the human moderators are overwhelmingly exposed to the trickiest, least cut-and-dry situations.
or refusing to delete photos of women and their ex-partners when they've been killed by their ex-partners.
http://www.bbc.co.uk/news/uk-england-gloucestershire-3461822...
It's impossible to know without watching something whether it's full of swearing or homophobic / misogynistic / transphobic hate speech.
Ads are not targeted to the actual viewer, but to some combination of regular browser user and main youtube user. this means that children will be shown ads for alcohol or gambling. (note this goes against what Youtube says here about complying with local law: https://support.google.com/adwordspolicy/answer/6012382?hl=e... Note that this is something all ad networks get wrong, and it's likely they're going to see regulation in the uk at some point if they don't fix it.)
It's hard to know before clicking a link whether it's sponsored content or not. (in the UK sponsored videos need to be clearly identifiable before the user clicks the link or clicks play. in the US sponsored content needs to be clearly identified).
If two people (eg a parent and their child) watch Youtube the recommendations fail hard. If a single person watches youtube the recommendations are pretty poor.