The broodlord has the hard to kill ability which means it essential take 2 wounds from a single shooting weapon attack to kill him. First this means that the flamer or the Liberian psychic storm power cannot kill him. It also makes life pretty hard on the stormbolter armed terminators who need double sixes to get him normally (2.8%) but a little better with sustained fire (11.1%). Now when it comes to overwatch the survivability of the broodlord really starts to shine. He has like a 70% chance of moving 6 spaces toward the overwatch marine. That is pretty good given that a normal stealer was usually toast after 2 moves without a jammed storm bolter. Here is the chart. Life is a little tougher if the space marine unjams the bolter but that action essentially gives you two squares of easy movement before the sustained fire returns to give its low but reasonable kill chance.
Definitely looks like a Broodlord can storm the castle.
Now the assault cannon is a different story. With 3 dice it is a lot easier to score two kills so you can bring the big beast down 33.3% of the time with normal fire which goes up to 50% with sustained fire. The assault cannon is the broodlord killer so stealer players should try to reveal him as from the assault cannon as possible while the space marine player needs to be bringing this big gun to bear on him. Here is the overwatch chart.
The broodlord should probably not move infront of an overwatch assault cannon unless the first squares of movement lets him assault it(ie there is 1 empty square between them) or will get him out of sight.