I’m going to go ahead and refute some of the counterarguments I’ve heard a million times over the years just to get it out of the way.
“It could be a while before it’s necessary.”
People seeking to upload and share unsavory content are constantly getting kicked off every other platform for doing so, and thus are always on the lookout for something new to try where they might be able to get away with it, at least for now. They are the earliest adopters imaginable.
“Just let users flag content”
Lots of issues here, but here’s a couple big ones.
1. You cannot afford something like child porn to be visible long enough to be flagged, or for it to be seen by anyone at all. If something like this gets uploaded and is visible publicly, you could be screwed. I worked on a video platform once that had been around a couple years and was fairly mature. One video containing child porn managed to get uploaded and be publicly visible for about one minute before being removed. It was a year before the resulting back-and-forth with federal agencies subsided and the reputation of the platform had recovered.
2. People uploading things like pirated content tend to do so in bulk. You might see people uploading hundreds of videos of TV shows or whatever. It may exceed legitimate uploads in the early days of a platform. You do not want to burden users with this level of moderation, and actually they aren’t likely to stick around anyway if good videos are lost in a sea of crap that needed to be moderated.
“We’ll just use (some moderation API, tool, etc.)”
Yes, please do, but I’m not aware of anything that works 100%. Even if you filter out 99% of the bad stuff, if the 1% that gets through is kiddie porn, say goodnight. These tools get better all the time, but users who are serious about uploading this kind of stuff also continue to find new and interesting ways to trick them. As recently as 2017 a pretty big video platform I worked on was only able to stop everything with a combination of automated systems as well as a team overseas that literally checked every video manually. (We built a number of tools that enabled them to do this pretty quickly.)
Content shouldn’t be moderated
Child porn? Hundreds of pirated episodes of Friends instead of legitimate user videos? (Even if you are pro-piracy, you don't want to pay to host and serve this stuff, and you don't want it to distract from legit original content from your users.) What about when some community of white supremacists gets wind of your new platform and their users bomb it with all their videos?
Do not take this stuff lightly.
EDIT: I've spent most of the last decade as an engineer working on UGC and streaming video platforms