Honestly the rules are not well spelled out. On one project we linked to other users content and displayed all of the ads associated which YouTube wanted to add, but when the aggregate amount of content delivered became too high our ip's were blocked. When we asked them they said we were out of the terms of use. There was no way to fix it. This was in 2014, by the way, so quite recently.

There is a substantial cost to video delivery. If you try to push this onto a service to take the cost for you, it really will backfire eventually.