Being a "rocket launcher" doesn't really describe the kind of damage it does.The alternatives are to nerf their power output which I don't really think it fair. It's a rocket launcher. It should be deadly. Just maybe not this deadly...I don't think more special rules and exceptions is the answer, but here you see how easy it is to resort to them for cool solutions
Is the rocket launcher you imagine more impressive than a cannon?
If we're talking about a BUK missle launcher, yes. It is more impressive than a cannon. Even if a single rocket might not do significantly more damage to a single target than a cannon ball would, 4 missiles certainly would.
I know which launcher the Infernal Rocket Battery is meant to emulate to my mind.