If you are going to tell people to avoid going to Masses at a "rival" group, you better have a good reason.
1. There's a big difference between "that other Mass is far inferior, because the whole group is compromised..." and telling people to STAY HOME IF THEY HAVE NO OTHER OPTION than attending Mass with that group. Big difference.
It's OK to have an informed opinion, and decide a good-better-best among the lifeboats of Tradition. After all, Tradition is inherently chaotic (the shepherd has been struck, the sheep are scattered, and no authority to sort everything out!) and a man can become well informed and form his own opinion on which lifeboat is safest to park his soul for the duration of this Crisis in the Church. That is what we all must do, using the virtue of prudence. Do our best to keep the Faith. We will disagree with some of our fellow Catholics, but we must maintain charity and patience with our fellow suffering, faithful Catholics.
The problem comes in the raising your own fallible opinion to the level of dogma.
2. If you're going to tell people to avoid going to Mass with a "rival" group, you also better have some REASONS. For example, the validity of the priest! That's a big one. No point going to "mass" if the Holy Sacrifice isn't in fact offered up, and the Blessed Sacrament isn't confected!
Or if the priest at the local chapel of your "rival" group is a danger to the virginity of your children, or to the fidelity of your wife. That would be a good reason not to go.
But when Fr. Pfeiffer (for example) says you can't attend Mass with Fr. Zendejas (for example), then "you lost me" as the saying goes.
When the reason boils down to, "He's my competition, and I'd really appreciate it if you stayed loyal to me, and expressed that loyalty by boycotting this rival of mine" -- that's just sick and anything but Catholic.