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: