I won't get into the details because I am simply an unqualified judge in the matter.
I have read several criticisms on the new rite of ordination of priests and/or the new rite of Episcopal consecration.
In all of these articles, those which claim the new rites to be invalid are compelling. If the facts that are alledged are, in fact, true, then the criticisms are almost certainly true--and the new rites are absolutely invalid. The problem is that the it is virtually impossible for a simple layman to independently confirm the facts presented. On the other hand, if there is any error in fact, the arguments are almost certainly invalid.
On the other hand, those which claim the new rites to be valid seldom address the opposing arguments and generally do not argue from facts but from feelings and faith that the new rites surely must be valid.
The rites of ordination and consecration is not a sacrament in which Christ gave the precises form and so the Church can make changes. The only question is whether or not the new rites truly do have a valid form, one that is valid in a way that the Anglican orders are certainly not valid.
All of this is beyond me. On the other hand, I believe it is enough that credible "experts" (I use the term loosely, because, on this matter, I'm really not sure who is and who is not an expert) disagree. The new rites may or may not be valid. It is also historically true that the new rites' validity was questioned by some theologians almost immediately upon their promulgation.
The possiblity of invalidity was one of the greatest motivating factors for Archbishop Lefebvre to consecrate four bishops for the Society. This one fact alone--that the archbishop did indeed consecrate four bishops for the Society--is enough for me to question the validity of the new rites and, to be on the safe side, avoid priests who have not received Holy Orders in the traditional Rite from bishops who have not themselves been ordained and consecrated in the traditional Rite.
If you still want the details, you should be able to find many scholarly arguments on both sides of the question on the internet. Unfortunately, there is not simple, easy to understand, explanation.