Mr Snuggles
It is partly the type of reverser (bucket/clamshell)but also how the airflow comes off the bucket and interrupts the airflow from the forward motion of the plane over the vertical stabilizer and the rudder.
oversimplification: relative wind (from motion of airplane ) moving over vertical fin at 100 knots. Jet Exhaust from bucket reversers moving from opposite direction at 60 knots , so resultant air flow over rudder is 40, not enough to do much. These numbers are for illustration only.
MANAADASYSTEM: fine, tell us what you would do in the situation you indicate. maybe you don't see what is happening? In the delta crack up, there was more reverse thrust on the left engine wasn't there? And the plane left the runway going to the left. Seems that reverse thrust can move the plane, just not used properly in this case.
And I've been to many slippery runways. I flew what EK Gann use to call AM21. Those funny airports on the top side of the US