I'm using that same service (AT&T prepaid unlimited) and it is unlimited data - but it can tell when you're downloading movies or big files via tethering and can throttle the hell out of your speed.
Videos that are embedded in web pages that don't allow you to view the video in the YouTube app pretty much won't play because of the throttling. If you can play it in the YouTube app, you can watch them all, but usually only at 480P resolution due to att&t's "Stream Saver" throttling (that you can't turn off with this package).
My usual download speeds are about 30Mb & upload are about 5Mb - in this town, but I'm a mile away from the tower. How good your connection will be depends on how close you are to a tower and how populated the area is using that service