If you'll forgive me for the tangent, but it seems Catholics are frequently held to an extreme double standard on this kind of matter. If Catholics aren't actively denouncing bad people in a bad society to the point of being firebombed or shot in public, they're "tacitly approving" of them. There's very ignorant people that still argue Pope Pius XII never denounced the Nazis
despite having a sermon read in every German Catholic Church as the homily on Sunday Mass in 1937 that antagonized the Nazis in every way possible.
Mit brennender Sorge was indeed an important and courageous document and did represent a major condemnation of Nazi ideology. However, it's important to recognise its limitations as well as its strengths. The encyclical criticised Nazism solely on theological grounds, not moral. The first five paragraphs are devoted to insisting that the church is not at all responsible for everything that's gone wrong since the signing of the concordat. The rest of it attacks Nazi ideology for elevating race to an idolatrous level, turning theism into pantheism, rejecting the necessity of Christ for salvation, rejecting the Old Testament as genuinely revelatory, and preventing the church from exercising all its functions. Much of it is concerned with the Nazis' use of Christian language, such as "God", "faith", "immortality", etc. to mean non-Christian things.
I don't find a single word in the document condemning Nazism on moral grounds, such as an insistence that antisemitism is simply wrong or the ways in which Jews had been treated up to that point was immoral. In fact there isn't a single condemnation of, or reference to, antisemitism in it. The closest it comes is paras. 29-30, where it says that without Catholic teaching, society will inevitably turn to immorality - quite a different thing from denouncing as immoral what it was already doing. This document postdated the Nuremberg Laws.
The impression I, at least, get from
Mit brennender Sorge is that if the Nazis had left the Catholic Church in peace, and not usurped Christian language to mean other things, the church wouldn't have been nearly so bothered. The overriding objection articulated in the document is that the Nazi regime is encroaching on the territory of the church, either doctrinally or pragmatically - not that it's harming others. I find this impression reinforced by the opening of para. 34, where it is condemning the fact that all young people had to join the Hitler Youth which taught doctrines inimical to Catholicism:
No one would think of preventing young Germans establishing a true ethnical community in a noble love of freedom and loyalty to their country. What We object to is the voluntary and systematic antagonism raised between national education and religious duty. That is why we tell the young: Sing your hymns to freedom, but do not forget the freedom of the children of God.
In other words, there's absolutely nothing wrong with forcing children to join a nationalist movement and sing patriotic songs against their will, provided that this movement doesn't also teach them a distorted version of Christianity or prevent them from fulfilling their Christian obligations at other times. In fact the main objection that this paragraph offers to the Hitler Youth is the fact that it prevents members from showing proper Sunday observance.
Of course the Catholic Church was not alone in failing to condemn National Socialism on moral grounds as well as theological. The Confessing Church criticised it for precisely the same things. Its
Barmen Declaration, mainly written by no less a figure than Karl Barth, offered pretty much exactly the same criticisms as Pius XI. Like the Catholic criticism, it was occasioned by Nazi government interference in the church rather than by horror at Nazi beliefs and practices in themselves. This was why Bonhoeffer, among others, broke with the Confessing Church, because he was frustrated at its failure to condemn Nazism on moral grounds. (On the other hand, the Barmen Declaration was written in 1934, not 1937 as
Mit brennender Sorge was, so one might be more inclined to excuse its authors somewhat on the grounds that the Nazis' actions had not yet become so obviously worthy of condemnation.)