I am cooking a brisket in a 12l container right now. It is packed absolutely full right now. Water covers everything, but pieces are pressed against the sides and against each other. Is this ok?
No, that doesn't sound good. The circulating water is what transfers heat efficiently to everything, making sure the water is the same temperature everywhere. If the flow is obstructed that can't happen. And if the food isn't all surrounded by water that's held at the desired temperature, it won't get heated as desired.