"Odd image. The Divine Mercy image is a strange imitation of the Sacred Heart without the heart. Christ is halo-less. The signs of his passion are almost invisible."

Gloria.tv asked "What Is Wrong with Faustina Kowalska’s Divine Mercy Devotion?":

Forbidden. What is wrong with Sister Faustina Kowalska’s Divine Mercy Devotion? Pius XII placed the Divine Mercy Devotion, including the writings of Sr. Faustina on the Index of Prohibited Books. John XXIII confirmed this prohibition twice. In November 1958, the Holy Office stated that there is no evidence of a supernatural origin of Kowalska’s revelations. This translates, “We do not think that these apparitions come from God.” The Holy Office declared that no Feast of Divine Mercy should be instituted, and it forbade to propagate this devotion and to publish the image of Our Lord as Divine Mercy. This was confirmed by a March 1959 decree.

Odd image. The Divine Mercy image is a strange imitation of the Sacred Heart without the heart. Christ is halo-less. The signs of his passion are almost invisible. This contains the claim that we can expect an unconditional mercy without the cross. The central error of the Divine Mercy is that it promises lots of spiritual rewards with no penance and reparation. Unfortunately, this corresponds very much with what John Paul II wrote in his misleading Encyclical Dives in Misericordia.

Concerns. The 640 pages writings of Sr. Faustina pose cause for concern. On October 2, 1936, Faustina stated that the “Lord Jesus” appeared to her saying that – quote – “I am uniting Myself with you so intimately as with no other creature.” Wait a minute, Our Lord united Himself more intimately with Sr. Faustina than with any other creature? So, Our Lord was more united with Sr Faustina than with Our Lady?..

... An author M M Anthony has written two books on this contentious issue:- amazon.co.uk/…nanswered-Questions-ebook/dp/B073DX8NX1/ref=sr_1_2

and

amazon.co.uk/…er-Approach-Getting-ebook/dp/B07PQLBD9S/ref=sr_1_2

I've long suspected that this devotion only escaped further censure because of the Polish connection with Pope John Paul II who rehabilitated it. I find it very man-centered - 'Have mercy on US' - as opposed to the God-centered Sacred Heart devotion, where we make reparation to HIM. The Sacred Heart visionary, St Margaret Mary Alacoque said: ”I understood that devotion to the Sacred Heart is a last effort of His love towards Christians of these latter times, by proposing to them an object and means so calculated to persuade them to love Him… This devotion was the last effort of His love that He would grant to men in these latter ages, in order to withdraw them from the empire of Satan which Jesus desired to destroy, and thus to introduce them to the sweet liberty of the rule of His love, which He wished to restore in the hearts of all those who should embrace this devotion.” (St. Margaret Mary Alacoque, vision of the Sacred Heart, 1673). If this was Our Lord's 'last effort' where does the Divine Mercy devotion fit in, all those years later?

I think it safer to stick with the Sacred Heart devotion. The definitive book on the Sacred Heart devotion can be found here:- tanbooks.com/…f-jesus-how-to-practice-the-sacred-heart-devotion/

The cover states that this book “is not ‘just another book on the Sacred Heart devotion’, but it can be truly said to be THE book on this devotion. Written by the spiritual director of St Margaret Mary, the book comes from the pen of a man intimately familiar with every aspect of the revelations given by Our Lord to this famous saint; thus the book is actually the ‘key’ to understanding the importance and the centrality of the Sacred Heart devotion for our lives as true Catholics. But even more than being THE book on the Sacred Heart devotion, Fr Croiset’s work is a revelation to us all just why so few people become great saints, just why so few Catholics – despite going frequently to the Sacraments – fail really to grow in the life of grace and make great progress in the spiritual order. ‘Devotion to the Sacred Heart of Jesus’ was actually commissioned by Our Lord Himself through St Margaret Mary, and as the author neared completion of it, the Saint told him that Our Lord said it was so completely in accord with His wishes, that it would never be necessary to make any change in it. Later she revealed to Fr Croiset that it was Our Lord Himself who had inspired him with the ideas in this book and that it was so pleasing to Him that ‘none other but Himself could have arranged everything so much to His wishes . . . ” [https://www.gloria.tv/post/DAyM8rrgEnVj1jgiJjebXES9f]