Agree the Greatswords underperform somewhat.
I think they're a min or max unit. Take a big horde (a points sink), or take several minimum size units.
Try smaller units with detachments. Try putting the GS into narrow formations, like 3 wide. The idea is to tarpit the enemy with a small disposable Stubborn unit for a turn or two, so you have time to maneuver elsewhere.
If you're playing T9A, minimum units of Imperial Guard with Sword&Shield are passable stoppers - again with detachments and other units for flanking duty. But they need a character to become Stubborn (the Bodyguard rule), and that makes them again very expensive for the job.
-Z