The biggest problem with enlightenment is that it inspires and makes possible those antienlightenment movements which are unavoidably, radically modern.
Maybe this is the true application of Hegel’s dialectical reading of history to the modern world, which Marx and Fukuyama both equally missed. Liberalism doesn’t lead to global communism or to a liberal order; it generates totalitarianism, modern extremisms, and can’t help it, can’t stop doing it. Simply by being itself, it brings its great antithesis into being.
Modernity promises increased wealth, truth, political representation. Each of these promises is fulfilled in general, but often not (or not equally) for specific groups within a society, which, in turn, leads to the modern reactions against modernity. Those modern reactions pretend to be scientific, true to the people, economically progressive, and the increasingly obvious fact that they are actually not any of these things is entirely unproblematic to their apologists, insofar as all the other modern political players are (as the extremists regularly point out) obviously just pretending as well.
Hannah Arendt helpfully insists on speaking of totalitarianism as a single thing, for which racism and communism are just convenient rationalizations. Fascism and Bolshevism are in one sense revelatory of the antienlightenment response, but they aren’t the fulfillment of it. Totalitarianism by its nature is not national (even if its rhetoric may sometimes be) but must by its internal logic always seek to be global, as Stalin and Naziism made clear.
Both extreme expressions (of the left and of the right) are illiberal, and both denounce the other side for being insufficiently liberal; each presents itself partly as an emergency defence of liberalism against an enemy, and partly as something which is superior to liberalism, the illiberalism of which is justified by the illiberalism and hypocrisy of the opposite side. Liberalism itself (eg Britain) is treated as merely a disguised instance of the opposite side.
The French Revolution obviously gives an early example of enlightenment politics, but it quickly shows the shadow side of those politics as well. It’s fair to say that the terror isn’t true to the ideals of the enlightenment, but it’s also fair to point out that it grows out of and draws its strength from what the enlightenment was doing.
Liberalism’s best defenders claim plausibly that if we can grow the economy fast enough then we will stay ahead of the demand for extremisms. The problem is that we can’t actually control everything (not even the reality, let alone people’s perceptions of reality), and so we can’t guarantee that there will never come a wrench into the gears that throws off economic growth for some length of time (eg extreme weather). And when that economic disaster happens, as it inevitably must sooner or later, the more technologically developed we are at that moment, the more powerful will be the political and social and economic tools of whatever group may be able to come to power.