Typically the problem is that Comcast won't peer with you. They always use the excuse that they only peer with equals, and since they aren't sending you as much traffic as you're sending them, it's not in line with their peering policy. This, of course, is a problem of their own creation; they're cable so customers all have a tiny amount of upload bandwidth and a large amount of download bandwidth. It is unlikely their policy permits peering with anyone. The ultimate effect is that you have to buy transit from a Tier 1 ISP instead of with the consumer ISP directly, costing you money. There are, of course, backroom deals where they sell a subscription tier that doesn't include video and then they throttle all the video. That's different than the peering issues; there is enough capacity to send all of your packets to them, they just throttle them on their end to squeeze money out of their customers.

I've worked at 2 ISPs and we obviously didn't have this peering policy, because it's dumb and it breaks the Internet. We also didn't throttle video, because it's dumb and breaks the Internet.