Anyone whose ISP/firewall/etc blocks all Torrent traffic is going to have a major challenge - which means that while part of what PeerTube has been so far is educational videos, they're not accessible in most school environments. That's a big challenge to overcome.
The other is pretty simple, and a reason I have not used PeerTube to watch a single video. If you're using my machine and my bandwidth to host or transfer your video to another person, you're going to pay me for it - upfront and at a rate I agree to. You're slowing down my connection and your website is causing me send data to someone I do not know. You're going to be liable for anything malicious that gets sent back; and if you're MitM'ing it to scan it properly, why are you wasting my bandwidth and storage?