But often they are more Catholic than the Pope... (Not that Martin Luther isn't either...).
While I would not say that the Novus Ordo is certainly invalid, like Traditio does, I would consider three main problems... (This is the Novus Ordo in Latin, with all attendent bells, smells, etc, and Gregorian Chant in the background... ie Maximum lipstick on this pig...)
1) The Novus Ordo has no offertory. No offertory, no sacrifice. Even satanists know that.
2) In the consecration of the Precious Blood, the formula defined by the Holy Council of Trent is nowhere in evidence. "This is my Blood of the New Covenant" is not sufficient. The words "Mystery of Faith", and "For Many" must also be present.
3) Now we have the whole question of the priests ordination, and boy, that is one huge can of worms opened by Pius XII with "Sacramentum Ordinis" back in 1947. And, even though the Novus Ordo has the dubious formula of Pius XII (which contradicts the Council of Florence) for priests, it lacks even that for the consecration of a bishop, so "Traditionalists" like Fr Pierre Marie are left with the arguement that the pope is the pope is the pope...
The mind of the Church is that Catholics are not allowed to receive or partake in dubious rites where a positive doubt exists... According to Leo 13th the minister must be duly fit to administer the sacrament and intend to do what the Church does (He can be a heretic and ascribe to all sorts of wacko theories but as long as he does what the Church intends the sacrament is confected ex opera operatis...)
Receiving Communion in the Novus might indeed be valid, but Communion is not the Mass, and the Communion Service is not part of the Mass. Many Catholics are under the false impression that a Mass is valid if the Consecration is valid. This is not the case, a valid Mass must have Offertory, Consecration, and Priest's Communion, which correspond to the offering, immolation, and consuming of the victim.
ANd that is my $ .02