The changes to the new rites of ordination for priests and consecrations for bishops constitute a position doubt, the church teaches that doubtful sacraments are invalid and you cannot receive them unless in danger of death.
So the indult doesn't have valid Sacraments or a valid mass (as no consecration takes place).
This is what sede priests would like you to believe, and some of them openly admit they have no principled reason for it. They just don't want you going to the Indult, as that is a "competition" for them. As if saving souls is primarily about competition. Anyway, your opinion can be answered in various ways, but one of the shortest and simplest is from Eucharistic Miracles. A study of Eucharistic Miracles, including those that have caused Atheist Scientists to repent and become Roman Catholics, proves that the New Rites are valid. They obtain less grace from God, but sufficient grace for the Sacrament to be valid. The infallibility of the OUM proves the same. For, if 5000+ Bishops of the Universal Church use and approve these rites, it is obvious they cannot be invalid. Anyone who says otherwise doesn't realize he has denied OUM Infallibility, which was defined at Vatican I. A study of the new rites shows that they retain the parts necessary for validity. They are valid, although it is permissible to believe the Traditional Rites are objectively superior and preferred by Heaven.