From the way I read the Sabotage description, I assumed that when the saboteur beat the DC by 5 or more, she not only left no sign of her breaking it (so that it doesn't work/falls apart when fired), but also was able to decrease the max range in the case that someone undid or fixed the damage she had done.
Example: I sneak into the orc base and slip my way to their rock lobbas. Not only do I rig them to not work (loosening frame bolts, untying ropes, etc), but I also manage to set the catapult to constantly undershoot irregardless of my other tamperings. So, when the evil gnome mechanic decides to do one last check of his babies prior to action the the next day and discovers my handiwork, even if he fixes the blatant sabotage, it won't be until they're fired that anyone will realize that I've messed with the range settings.
In other words, Sabotage's ability to decrease range acts as a backup to the outright tampering. When the enemy combat engineer/mechanist/repairman undoes the tampering or does some on-the-spot battlefield maintenance (after the frame collapses or the rope snaps), the range is still messed with until that particular piece of sabotage is fixed.
Also, I assumed that the "more complex devices" meant that the Saboteur could disable Siege Towers, Trebuchets, Repeater Bolt Throwers, etc. Most catapults/rock lobbas or ballista/bolt throwers are pretty simple in construction and design, whereas a trebuchet or repeater ballista works off of more complex engineering principles.
Please take note that nothing says that the saboteur also can't gimmick with crossbow range increments (a ballista-in-minature).