Yes, the Dimond brothers are a mixed bag.
Some of the work they do is tremendous. Their biography of Padre Pio has to be one of the best I've ever read. They wrote this pamphlet "How the Bible Proves the Teachings of the Catholic Church" that's a masterpiece of apologetics. Their stuff on UFOs was tremendous. They have done absolutely terrifc work showing the problems with evolution ... from a scientific standpoint. Their video on "Hell" is very motivational. And they practically give their materials away (most of it is available free online, while the rest they must be taking a loss on). I consider them to be right on a number of issues, e.g. NFP and MOST of their thinking on EENS.
Where they crossed the line, however, is in having gone dogmatic both with regard to BoD and with regard to Sedevacantism. To call even CLASSIC BOD such as held by St. Thomas Aquinas "heretical" and to label non-sedevacantists as non-Catholics crosses a very dangerous and very serious line ... into a schismatic mentality. Sometimes they fail to make the appropriate distinctions and make everything a dogmatic black-and-white. It's this dogmatic stuff (which they have more in common with the dogmatic SVs than the classical Feeneyites, most of whom are R&R or in communion with Rome) that has caused them to go off the rails.