It isn't the mixed member, as such, that creates coalitions, but the fact that it is a proportional system. The additional member component completely compensates, at least within the realm of arithmetic, for any disproportion at the constituency level.
I think the local representation component of electoral representation in the country is an oversold argument. One of the appeals of MMP to me is that you could have largish rural constituencies. You could have a disproportion that let's Nathan Cullen represent half the population of a typical Surrey riding. You could do that and fully compensate through the compensatory tier.
As for the question of the accountability of coalition governments, I think there are many systems in which they're perfectly accountable, and the focus is on the nucleating party, the one that is the formateur in the system. That's the one that, in some sense, takes the hit. There are coalition systems in which it does break down, because typically there's some pivot that is off-scale and extracts inappropriate equivalents.