  We analyze the transformation of QUBO from its conventional Boolean
presentation into an equivalent spin glass problem with coupled $\pm1$ spin
variables exposed to a site-dependent external field. We find that in a widely
used testbed for QUBO these fields tend to be rather large compared to the
typical coupling and many spins in each optimal configurations simply align
with the fields irrespective of their constraints. Thereby, the testbed
instances tend to exhibit large redundancies - seemingly independent variables
which contribute little to the hardness of the problem, however. We demonstrate
various consequences of this insight, for QUBO solvers as well as for
heuristics developed for finding spin glass ground states. To this end, we
implement the Extremal Optimization (EO) heuristic, in a new adaptation for the
QUBO problem. We also propose a novel way to assess the quality of heuristics
for increasing problem sizes based on asymptotic scaling.
