It's pretty amazing how well that scales, really. What surprised me the most was, that it also works well for live streams - I witnessed that in action when heise (that huge German tech publisher which recently created their own instance and you may have seen a lot in the trending videos) streamed their live show for the first time. Even on a live stream, towards the middle when I checked, I had more downloaded from peers than from the central server, and more uploaded than downloaded.
I also have my instance set up to mirror a lot of videos from trustworthy, popular instances - and even so, traffic statistics show no issues whatsoever so far, if something really does attract lots of concurrent viewers, the p2p feature handles that quite efficiently.
Now, if only storage was trivial in cost, too, costs for running PeerTube would basically be the same as for text/image-focused Fediverse stuff.