That was in Dark Crusade right? Luckily in Soulstorm Broadsides are pretty easy to kill.
Not really ... my Broadside took down about 9 Space Marines. Granted, they weren't upgraded, but still ..
I found it quite amusing to see on the titles of some DoW multiplayer games "No Tau." Fire Warrior massing is like the most annoying thing in the universe. 
I don't see whats so annoying, tbh. My friend tried that on me, and I just dropped in Flayed Ones and the Fire Warriors dropped like flies.
How does that work? 
Watch the Broadsides fire in Dark Crusade. It looks kinda like a guiding beam that the shell uses to travel to its target. Like the UCS plasma cannon superweapon in Earth 2140 series, which fires a plasma beam, which does nothing, merely transporting its payload of bombs down the beam.