Let's have a more objective look at the Dimonds, since most people have an ax to grind.
1) I agree that they are not good debaters and do not construct solid arguments. In particular, they fail to see finer distinctions that would make certain points of theirs more complex than they pretend.
2) They often miss subtle nuance in some of the quotes they adduce to prove their positions.
3) As with the dogmatic sedevacantists, this lack of ability to consider complexities and to make distinctions leads to a dogmatism, a pretense that their position is essentially de fide because it derives from a simple obvious "irrefutable" syllogism. They use the word "irrefutable" all the time.
4) They do have a lot of talent and do much good. Their life of Padre Pio, their book on UFOs, their video on hell, and their How the Bible Proves the Teachings of the Catholic Church, their materials exposing evolution and modern science, et al. are absolutely wonderful.
5) They practically give their stuff away, likely at a loss (charging just pennies for books, videos, and pamphlets); they are not about making money.
6) In terms of their being brothers, the charge that they are "fake Benedictines" is absurd and uncharitable. They're every bit as REAL as 99% of all Traditional groups of religious that have been founded without the requisite jurisdiction.
7) Whether any of you like it or not, the Dimonds have correctly identified the root cause of the problem behind Vatican II ... a heretical new ecclesiology that resulted from a gradual rejection of EENS. How do you get rid of EENS? Just redefine "Church" and "outside", and you're good. You're also now right there at Vatican II.
8) Dimonds then realized that the weapon used by the heretics to undermine EENS and create the new ecclesiology was BoD. But, unable to make the appropriate distinctions, they went overboard in declaring BoD "heretical" ... not realizing that one could defend EENS without necessarily rejecting BoD (at least logically). In fact, the ardent proponents of BoD ALWAYS conflate the issues on purpose because then they can pretend that they have the authority of St. Thomas, St. Alphonsus, etc. behind their refusal to accept EENS and their new gnostic-Pelagian-Protestant ecclsiology.
9) Dimonds recognize the bad will in most BoD proponents. Most of them hold to their opinion because the find EENS unpalatable and won't listen to reason. Consequently, the Dimonds became more frustrated and angry and bitter over time and have been overcome with bitter zeal.
10) In excommunicating proponents of BoD, the Dimonds have adopted a schismatic stance. I have repeatedly called them out on this. Father Feeney and the vast majority of "Feeneyites" do NOT do this but consider their position on BoD to be personal opinion.
What's interesting is that most of the bad traits of the Dimonds are not shared by the Feeneyite population as a whole, most of whom actually tend to be R&R. In fact, there are relatively few sedevacantist Feeneyites, and the most hostile enemies of Father Feeney and EENS come from the ranks of the SVs.
So most of the flaws one sees in the Dimonds are shared with the sedeavacantists and their bitter zeal and all its bad fruits actually emanate from the dogmatic sedevacantism rather than from Feeneyism.