The Inlines NEED the counterweights to help them RUN SMOOTHER along with the special fluid engine mounts, counter balance shafts and what ever else, to keep the viberations from the driver pants.
The flat 4's ARE SMOOTH WITHOUT THE ADDED gizmos and VW did not feel the need for counterweights.

AND...

Remember that ALL auto makers DESIGN THEIR CARS TO WEAR OUT!  Some take longer than others.  On the WBX engines VW knew that the WBX engines would fail around that mileage.  They wanted you to buy a new VW or get the engine fixed.

Fuel tank design was changed to cause them to wear out SOONER because not enough were being sold.

This is sililar to the problem in the T1 engines.

Engine that is SMOOTH as glass at 5600 rpms (and quiet) WITHOUT any gizmos to keep it smooth

Robert