Weakest link in any IRS are the differential mounts.  Unlike a solid axle, the differential housing in an IRS  is mounted rigidly to the sub frame via its cast aluminum housing and rear cover.  It must be rigid enough to withstand extreme twisting forces on all three axis, with flexible jointed axles instead of solid  rigid axle tubes.  But its also the most significant source of cabin noise when solid mounts are used (moaning and gear whine).  Factory mounts always err on the side of NVH and allow the housing to move too much, and that is how you break diff covers and axles with any meaningful amount of torque and not enough grip.
Its actually not good grip and power launches that break them, it's lack of traction.   The massive jack hammering that occurs against the soft diff mounts when a wheel slips and alternates between slip and traction as the LSD clutches engage and disengage rapidly, that's what wheel hop is. 
There is no excuse for squishy mounts in something like my Cobra, but in a Caddy there is obviously some expectation of a cush quiet ride.