People should avoid the Masses (and sacraments) of a compromise group, e.g., the N-SSPX or another indult group or “motu proprio” group, or Bishop Williamson’s group, or the sedevacantists. This is true even when that group has valid Masses and sacraments.
The Mass and the sacraments are good in themselves and are themselves causes of grace. However, this does not mean that valid Masses and sacraments always cause grace. For sometimes they involve compromises and defects which are sins and are obstacles to grace.
The end does not justify the means. So, we cannot use sin (a bad means) to obtain grace (a good end). When it is a sin to attend a particular group’s (or priest’s) Mass or sacraments because of compromises and errors, then those Masses and sacraments cannot do us good because the end does not justify the means and God does not give us grace through our sinning.
When a Mass or sacrament is from a compromise group or is otherwise (objectively) sinful, then no one should attend or receive the Mass/sacrament. Let us not attend any compromise Masses or receive (or attend) any compromise sacraments, telling ourselves that we “need our sacraments”!
God knows better than we do what we need and provides for us better than we understand. “All things work together unto the good for those who love God.” Romans, 8:28.
 For many articles showing that the SSPX is a compromise group, read the articles in the Society of St. Pius X folder found here:
 For twenty reasons the indult (“motu proprio”) Mass is objectively sinful, read this article:
 For many articles showing that Bishop Williamson’s group is a compromise group, read the articles in the Bp. Williamson folder found here:
 The sedevacantists are inherently in schism from the Catholic Church, although some of them might not know better and so might not be culpable for their grave sin.
 The Sacraments are instrumental causes of grace (“ex opere operato”). See, session VII, canon VIII. The Council of Trent distinguishes (on the one hand) the seven Sacraments – which cause grace – from other good works and prayers (on the other hand) through which we obtain grace, which are not themselves causes of grace. Reciting a Hail Mary is not a direct cause of grace. Rather, it is a pious occasion which disposes us and prompts God to give grace – but not through that prayer as a cause.
 One obvious example of this is a satanic Black Mass which, even when it is valid is still sinful and does not give grace. For further information concerning why a person might not receive grace from a valid mass or sacrament, read this article:
[S]ome have contended that heretics, schismatics, and the excommunicated, who are outside the pale of the Church, cannot perform this Sacrament [viz., the Holy Eucharist]. But herein they are deceived, because, as Augustine says (Contra Parmen. ii), it is one thing to lack something utterly, and another to have it improperly; and in like fashion, it is one thing not to bestow, and quite another to bestow, but not rightly. … [S]ince the consecration of the Eucharist is an act which follows the power of order, such persons as are separated from the Church by heresy, schism, or excommunication, can indeed consecrate the Eucharist, which on being consecrated by them contains Christ’s true body and blood; but they act wrongly, and sin by doing so; and in consequence they do not receive the fruit of the sacrifice [viz., grace]….
Summa, III, Q.82, a.7, Respondeo (emphasis and bracketed words and letters added).
For a further analysis of this issue, read this article: