Could you quote the part of the Concordat that you find to be objectionable?
newadvent.org/library/docs_ss33co.htm
Such Concordia are quite common whenever their is a change of government.
I certainly will:
Article 30
On Sundays and Holy days, special prayers, conforming to the Liturgy, will be offered during the principal Mass for the welfare of the German Reich and its people in all episcopal, parish and conventual churches and chapels of the German Reich.
Saying special prayers for the “welfare” of a political regime conveys, at the very least, an attitude of passivity towards the regime. The unfortunate fact is German priests were celebrating special masses for Hitler’s birthday up until close to the end of the Reich. The Concordat set this tone.
Article 32
In view of the special situation existing in Germany, and in view of the guarantee provided through this Concordat of legislation directed to safeguard the rights and privileges of the Roman Catholic Church in the Reich and its component States, the Holy See will prescribe regulations for the exclusion of clergy and members of religious Orders from membership of political parties, and from engaging in work on their behalf.
This in effect ended clerical opposition to the Reich, as I stated.
I understand that concordats with secular governments were common, and did not
necessarily equal an endorsement. It is not so much the fact that Rome executed a concordat with the third Reich, but the circumstances surrounding it and the effect it had which are problematic.