Stoddards Atlanta used to have a sheet with rounds they fired into ballistics gel. They also showed the before and after weighs. The Barnes Tac-XPd had great expansion and 100% weight retention. I switched my 9mm carry to them.
Gimmick mostly. The ballistics tests are often botched by folks that do them on a limited basis. Expansion and weight detainment are not as necessarily intertwined as they present. Some fragmentation can be a good thing if penetration is sufficient. Fragmentation can make trauma treatment much harder because bleeding is more difficult to control potentially.
I think the solid copper ones shaped like a phillips screwdriver on the end have the potential to penetrate body armor, especially in 9x25 Dillon, since they do not expand (and expansion is a means that body armor uses to stop a round). That's a 10mm round necked down to 9mm. There are conversion barrels you can drop into a Glock 10mm.
Something to consider if terrorists wearing armor try to shoot up a place. If they are 50' away you might not be able to get a head shot, so send several 9x25s their way, and that should stop them, or at least impact enough to take the wind out of them.