One has to be careful reading anchor "tests". Bottoms are different--even in the "same area"--once you drag a single anchor thru an area, the bottom is changed. It is not uncommon for two boats, with the same anchors and seemingly the same technique; one anchor sets, the other doesn't.
Also one has to be a bit cautious, because various people are pushing their anchors. The use of "efficiency" graphs to promote a single type of anchor can be misleading--again, different bottoms, and different holding characteristics. Peter Smith has been in a "battle" with Manson Supreme, since it may or may not have been copied from his Ronca design. Unfortunately Ronca suffered from some bad metallurgy and galvanizing. Manson early had some problems with the tip (welded two piece rather than single piece to give a heavier tip. Some anchors put lead in the tip to give better penetration.
One of the older studies--and unbiased by US sailing in 1994 and 1995:
http://offshore.ussailing.org/Assets/Offshore/SAS+Studies/anchor+study.pdf
http://offshore.ussailing.org/Assets/Offshore/SAS+Studies/1994+anchor+test.pdf (not quite as good a study as the one above which was a year later and to help rectify the irregularities in this study) One feature which repeats during both of these studies, is that the Fortress remains at the top of anchors, in the right conditions. So it still is high on the list of kedge anchors, and stern anchors as well as a back up storm anchor in the large sizes.
I spent most of this afternoon trying to find unbiased studies of the new generation of anchors. I could not find one which did not seem biased and included all of the newer anchors. Some which have not been mentioned are Anchor right (SARCA, Excel, etc) Manta (US company which has produced many videos where their anchor is the only one which holds--I don't believe this, but there are some good antidotal reports of good holding for this anchor. It bolts into 3 pieces. I am not sure why bolts are used vs welding--in the small anchors--in large anchors it make sense. The Spade, the Ultra, There is a long list of other anchors tested along the way.
Specifically I was looking for the reports on the Manson Boss. I did find two reports of the Manson Boss shackle preventer (the bolt which keeps the shackle at the business end of the shank, instead of sliding down the slot, and allowing the anchor to pull out if the pull was reversed. It appears that both failures of this bolt were when the anchor was being brought aboard in boats and sheared off when hitting the bracket of the anchor roller.
Although Manson says that the "Boss" is designed for power boats, it is finding a lot of popularity on sailboats where the roll bar (both it and the Ronca) was causing problems on the bow sprit. (I would have had this problem on the 62 foot ketch we took Calif. to Europe and back. I carried a 75 Lb CQR on one side of the bow sprit, coming thru the teak grating on the Stb side and a 60 lb Danforth High Test on the port side.)
I agree that I have concerns about the "slot"--on several occasions I have tied trip lines to the crown of my anchors when in areas where the anchor might hang up. Once a commercial boat ran over it , and tripped the anchor when they wound the trip line in their prop. Another time, a boater tried to pick up the float for a "mooring" even though the float was clearly labeled "Trip line".