I am a fan of Diderot by way of Rousseau. And his novel was interesting in the letter based structure. It quite forward thinking for its time, in that while the Enlightenment was directly challenging many statistic and institutional ideas, very few Enlightenment writers were including women in their considerations.
That said the user reviews here and especially the professional reviews are a bit overwrought as a result of leaving out context. Firstly almost all people were locked into vocation, virtually never of their own choosing, as everything about their life. Certainly married women were, and subject to control and violence that makes anything in "The Nun" pale by comparison. So too was just about anyone else for most of human history. Either overtly owned or tied to the land and their "station' and subject to warlord or state violence for for chalking that.
Certainly in mid/late 18h century Nuns were eating better (no small thing in world were people regularly starved to death), were safer in almost every way than most other people, certainly than the great majority of men, who were much more likely to be inducted into the military as cannon fodder.
Again, the 1966 version is better, and better yet is the novel.