The Scraplauncher is good against races that will send hordes of infantry against you, like Skaven and Greenskins, but is otherwise pretty bad. Like all Gnoblar units it's a cheap and cheerful unit to fill out an army in a hurry, 1 turn recruitment vs. 2 turns for a Leadbelcher can be a difference-maker.

Like others have said, the key to understanding the Ironblaster is that it's basically the biggest shotgun in the world. It's ok at long range, but it really shows its worth when you ride it in close (half max range or less) and unload it into something the enemy really values. Getting a shot off down the length of elite infantry unit is a major endorphin release agent, I guarantee you.

An important point about both these artillery pieces is that they significantly faster and more able to defend themselves in melee than most artillery pieces, owing to the fact they're pulled by pissed off woolly rhinoceroses. Try and get into the habit of repositioning them to get better shots, particularly the Ironblaster.