He is the author of Humanae Vitae and this alone should earn him sainthood given the mentality of the day which hasn't really left the Church until today. This one encyclical brought him more grief from Catholics who lost faith in natural law and turned to the law of medications to limit births which opened the door to pro-choice Catholics.
Did Blessed Paul VI make mistakes? Yes, but to err is human.
These are the major mistakes Blessed Paul VI made:
1. He set up a committee headed by Bishop Bugnini to revise the Mass and gave him too much authority to revise the Mass not as the Second Vatican Council asked but the way that academic liturgists wanted.
This led to the dumbing down of the catholicity of the Mass to promote ecumenism. This dumbing down led to the loss of Catholic spirituality in the Mass especially with the wrong headed method of translating the official Latin of the new Mass by using dynamic equivalence rather than a literal translation done well. This affected English the most.
Unfortunately too, Pope Paul allowed for experiementation in the Mass as it concerns the implementation of Bugnini's reforms especially in the area of inculturation which has since made the Mass so different from parish to parish, that these Masses are radically different from each other compared to a Latin OF Mass celebrated with chanted propers and ad orientem and the Tridentine Mass which are more similar in this form.
2. Pope Paul lamented the problems that Vatican II caused which he believed were not intended but a result of the "spirit" of Vatican II but he was too weak to actually crack heads and fire bishops who went to far--there were no excommunication by Paul VI.
He lamented the smoke of Satan entering the cracks of the Church but that's all he did. He seemed impotent as a leader especially by the mid to late 1970's. He was depressed about many things.
My final comment has to do with my own attitude about Paul VI in my late teens and early 20's. He was weak and ineffective in guiding the Church, meaning giving proper leadership. He dour and ineffective in his latter years.
He made suggestions about corrections but didn't back it up with mandates, and in this way is somewhat similar to Pope Benedict who modeled what was right but didn't require it, like Communion kneeling and receiving on the tongue, ad orientem, more Latin and the other things that Pope Benedict desired for the Church in terms of renewal in continuity and reading VAtican II in continuity with what preceded.
No saint is perfect except in heaven and neither was Pope Paul VI perfect by any stretch of the imagination but he is in heave where perfection is attained and he prays for his successor, Pope Francis who isn't perfect either.
But Pope Francis has teeth and uses the authority of the papacy to get what he wants. The only problem is that what His HOliness wants isn't helpful for the Church today unless we accept a Protestant form of the Catholic Church.