I agree with you but i guess everything heavy armoured with few wounds, this mean heavy infantry/cavalry, war machines, other flyers. Even some monsters with T5 but only 3 wounds could be a good target. I haven't tested them a lot but still think they are better than cold one cavalry (than i tested and failed due both low initiative low wounds in comparison to ripperdactyls for their cost points). If you don't play champion in them (and I wouldn't) you can also try to reach other infantries with characters inside and try to kill them (8 ripperattacks + 2 from skinks)
Looking at your Old Bloods, I would put the ogreblade on the talisman of protection old blood and BORT on the armour of metoric iron. BORT is a better all rounder weapon, so charging into heavy cav and stuff, with potentially a lot of AP attacks but not multi wound stuff. Armour of Metoric Iron is good to have there, and the BORT will kill them easily. Ogre Blade is for killing monsters and dragon, where having the double save is a bigger deal IMO. Sure the 4+ can and will go do to a 5+ or 6+, but the when you pair that with a 5+ ward, that is still better chances over all. And the Ogre Blade will kill those things faster. Just my thoughts.