  Different methodologies lead to order-of-magnitude variations in predicted
galaxy merger rates. We examine and quantify the dominant uncertainties.
Different halo merger rates and subhalo 'destruction' rates agree to within a
factor ~2 given proper care in definitions. If however (sub)halo masses are not
appropriately defined or are under-resolved, the major merger rate can be
dramatically suppressed. The dominant differences in galaxy merger rates owe to
baryonic physics. Hydrodynamic simulations without feedback and older models
that do not agree with the observed galaxy mass function propagate factor ~5
bias in the resulting merger rates. However, if the model matches the galaxy
mass function, properties of central galaxies are sufficiently converged to
give small differences in merger rates. But variations in baryonic physics of
satellites also have dramatic effects. The known problem of satellite
'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites
are too efficiently stripped of gas, could lead to order-of-magnitude
under-estimates of merger rates for low-mass, gas-rich galaxies. Fixing the
satellite properties to observations tends to predict higher merger rates, but
with factor ~2 empirical uncertainties. Choice of mass ratio definition
matters: at low masses, most true major mergers (in baryonic/dynamical galaxy
mass) will appear to be minor mergers in their stellar or luminosity mass
ratio. Observations and models using these criteria may underestimate major
merger rates by factors ~5. Orbital parameters and gas fractions also introduce
factor ~3 differences in amount of bulge formed by mergers, even for fixed mass
ratio encounters.
