Yep, throwing hammers are pretty sucky.
Even after the greater damage bonus from heirlooming to +3 (4b >> 4c), they do one or two points of damage after armor vs 55-70 armor when compared to HTA.
Before heirlooming, they do about the same damage vs 55 armor, but the hammer has worse melee/projectile speed/throwing speed/weight, no bonus vs shields (even if HTA didn't have bonus vs shields, it would still do more vs shields).
And obviously, they perform worse vs squishier targets (<~45 armor?).