Government will bring tyranny regardless of the race uniformity of the people. Just look at China. The lack of trust due to a mixed people is just an excuse, but government wants authoritarian power regardless.
Discussion
True, they can bring tyranny and often do, even in countries lucky enough to have escaped multiracialism. But it's much, much harder to unite and launch a revolution against tyrants if your fellow "citizens" are of 100 disparate backgrounds. And it's much, much easier to (mis)rule and oppress a squabbling congerie of tribes than a united singular people.