Galen's excellent critique tells the story, but I'll 'splain it :
The B&M's four bolts, bunched as they are along the back edge of the adaper plate, concentrate all
of the mechanical force attaching the shifter in one small, off centered, area-- well behind
the shift hinge point.
Hard 1-2 and 3-4 shifting imparts a force vector that is way
the hell out of the shear plane, as it attempts to peel the front of the adapter plate away from the tranny housing. The force is at least 10-15 degrees off plane, in tension, headed down and back. Unlike the bolted-direct-to the housing GMM, the B&M re-used the OEM tansmission adapter plate which in turn
bolts to the tranny housing. The tranny adapter plate, sitting as it does on the B&M-supplied higher-durometer-than-stock gasket, is still free to flex and move under this force.
Facts is facts: for a measly $100 more you can have a shifter that is better built and better designed, mounting in such a manner that eliminates
the tranny adapter plate altogether, getting rid of one inevitable
Mounting issues aside, the GMM has an internal gate and detents that makes shifting far less sloppy and way more positive than the B&M does.