What would handling it “well” mean?

Not having populist demagogs reach significant power would be a nice start. Though I'm not sure if the migrant "crisis" is actually the cause or just a symptom for deeper issues.

Not dividing population into two sides on the refugee issue, not breeding so high level of internal resentment. Having a bit more united front on the issue. I'm half-convinced that the way the current refugee crisis ended up being handled was much more disruptive to our internal politics than just letting them all in would be to our economies.

Handled it pretty well imo

Well in the sense that it didn't end up being a comparably large humanitarian crisis on our borders, and that we didn't break the Union into pieces (though the tensions created may well lead to that).

Not well in the sense that when the same thing happens again with climate refugees, only 10-100x the size, I don't see us surviving.